Ensure that all *.*proj, *.props, and *.targets files are prefixed with the xml and copyright elements

Fixes #15910
Also fixes a sequence point issue with the let expression. Specifically, the source range for the query lambda included more than the expression of the let, but included the entire query clause. It now includes only the expression, like other query clauses.

Fix our use of suppressParent in project.json

On machines with Dev15 installed there are additional DLLs in the set of
assemblies that can be referenced by default.  The SDK adds them by
default.  This introduced a new confilct that needed to be resolved.

TLDR: This change removes our use of suppressParent which should not be used

The core problem in our build setup which lead to the use of suppressParent is having
the same reference DLL introduced by different NuGet packages.  These packages differed
in name and version from each other.  The most notable example is the reference DLL for Microsoft.VisualStudio.Language.NavigateTo.Interfaces.dll which is contained in 3
different NuGet packages:

- RoslynDependencies.Microsoft.VisualStudio.Language.NavigateTo.Interfaces 14.0
- Roslyn.Microsoft.VisualStudio.Language.NavigateTo.Interfaces 15.0 preview 5
- Microsoft.VisualStudio.Language.NavigateTo.Interfaces 15.0 RC

The differing names is a problem because when resolving conflicts NuGet doesn't consider
the referenced DLL versions at all (by design).  Instead it is only concerned with
handling conflicts between packages of the same name.

These packages have different name and hence NuGet never attempts to do any conflict
resolution between them.  It will consider each package to be a separate entity and
pass on their assets to MSBuild.  This means that MSBuild will eventually be handed the
same DLL with 3 different versions and consequently begins issuing MSB3277 errors.

The suppressParent entries in the project.json file suppressed this error because it
essentially removes the listed package from the transitive graph.  Hence it never appeared
in referenced projects, only a single DLL was passed to MSBuild and compilations
progressed.

This is the wrong approach to fixing that problem because it's subverting both the
depenedncy conflict resolution aspects of NuGet / MSBuild and causing us to create
incomplete deployments in our unit test directories.  This is fighting the tooling
instead of leveraging it.

The more robust approach to solving this problem is to have a reference DLL always
distributed through the same NuGet package.  This allows NuGet to handle the version
conflicts using standard conflict rules and resulting in only a single DLL being passed
to MSBuild.

In the past this has been a blocker because we often need DLLs at versions that aren't
available on NuGet.  Going forward we will be working with the VSSDK to remedy that
problem.  Short term though we are simply going to upload ad-hoc packages with the correct
name to the roslyn-tools feed using the pre-release moniker -alpha.  This ensures we
don't have any conflicts with official packages on NuGet.org.

There are a few cases this change doesn't completely address that I want to call
out:

- Types moved between the MS.VS.Shell.Immutable and MS.VS.Shell.Framework DLLs between
  Dev14 and Dev15.  To prevent a lot of duplicate type errors the MS.VS.Shell.Immutable
  DLLs need to be removed from the compile graph, but not the runtime graph, in our
  Next projects.
- GraphModel is an adhoc package created by us that doesn't have an existing NuGet
  package to pattern off of and it's not obvious how such a package would be
  laid out if it existed.  The Dev15 packages also include this as a reference by
  default which causes a confilct with our packages.  As such I've used "include: none"
  for now to work around the problem until a final NuGet package is decided on.

Added a test to validate that __refvalue is not allowed to be returned by reference

Improve performance of code-style analysis.

Cleaning up the Roslyn build configurations

Better codegen quality when emitting degenerate switches.

Related to dotnet/coreclr#7914
Where the offending pattern was observed in the IL generated for the Kestrel web server and was recommended as an easy improvement.

Becasue of cascaded dispatching into "try" regions , it is not uncommon to see async state machine to contain degenerate switches like

IL_0008:  switch     (
                          IL_003b,
                          IL_003b,
                          IL_003b,
                          IL_003b,
                          IL_003b,
                          IL_003b,
                          IL_003b,
                          IL_0729)

Note numerous cases all leading to the same target.
We can trivially emit such switches as just a range-check (that switch would need to do anyways), without any actual "switching".
Since all cases lead to the same label anyways, there is no need to switch once we know the value is in range of the switch.

Fixes:#14878

Disable the vs-integration tests from running with PRs by default.

Fix a crash caused by an improper reuse of lambdas from return inference cache.

Ensure that CPS property setters also invoke the code within ExecuteF…

Similar change as to TestUtilities.  Insteading of having two
independent projects with shared sources there are now a portable and
desktop only version.  The desktop version references the portable one.