Also fix the IDE helper to get value usage info, which fixes the MakeFieldReadOnlyAnalyzer

Fixes #29373
Fixes #30175

Primarily addresses https://github.com/dotnet/roslyn/issues/29519#issuecomment-420067012

```
1. Enable the rules be default
2. Remove all the workaround/hacks for disabling the rules for product and enabling for tests.
3. Address offline feedback from @sharwell about avoiding deriving the tests from `TestBase`.
4. Separate out the pending non-design related work items from first comment into separate issues.
```

For 4., I have repurposed https://github.com/dotnet/roslyn/issues/29519 to track the pending issues with checkboxes in the first issue comment.

* Move xunit package references to Roslyn.Toolsets.Xunit.targets

* Remove dependency on Microsoft.NET.Test.Sdk

VB's behavior for If(Nothing, t1) diverges slightly from C#, as a
fallout of how VB handles If(Nothing, 1). C# doesn't let the code
compile, but if the unconstrained type parameter is a value type it
would take the first argument always. VB allows If(Nothing, 1), and it
will pick the second argument to the If. So, the unconstrained T
behavior matches this.

The cause of the CoreClr crash appears to be VB expression trees. I've
disabled all of the relevant tests util we can get a fix from the
CoreClr team.

I'm as confident as can be that this will be stable. I ran the test 100
times in a loop locally and got no failures. If more popup I'll dig
further

CoreClr tracking bug: https://github.com/dotnet/coreclr/issues/19862

Fix Tuple Crefs causing null exception:
  - fix the Cref binder so that we un-wrap any TupleTypeSymbols that get wrapped inside BindNamespaceOrTypeSymbol as crefs don't consider the higher order form tuples at all.
- Add tests

* Fix compiler server mismatched version:
- Ensure compiler server returns correct response when providing an incorrect protocol value
- Add test to check
- Fix tests that break

* Check compiler sha as part of build server protocol:
- Add the CommitHashAttribute to the needed assemblies
- Bump the protocol version to 3
- Add a new CommitHash field to build request
- Fill out the CommitHash from the CommitHashAttribute value when creating build requests
- Check that the commitHash on a de-serialized build request matches the one compiled into the assembly
- Return a new IncorrectHash response if hashes fail to match
- Fail early on the client side if we can't get the hash

Fixes #29481.

* NuGet packaging

* Add missing VS package references

* Fix scripts

* Add comment on Workspace.Desktop references with PrivateAssets

* Fix Microsoft.NETCore.Compilers package

* Fix up references

* Build fixes

* Correct OutputPath for package projects

* Use NuGetRepack to force exact versions in per-build packages

Fixes #29291

Add explicit case clause(s) for all the bound nodes for which right now we fall back to creating operation None node. Throw from default case.

This fix allows the rsp file to be used on case sensitive file systems like Linux.

GetCorrespondingLoop and GetCorrespondingSwitch have been merged into a single
extension method.

Yet to come - 1. delete spurious extension methods, 2. rework unit tests.

The icacls command is failing on Helix hence disabling these tests there
for now

Related to #28836

When conditionally skipping tests for CoreClr I mistakenly
unconditionally skipped them in my attempt to document the reason for
the skip. The use of the Skip property causes the test to be
unconditionally skipped, not documenting the reason.

In addition to fixing this I prevented this from happening in the
future. It's very rare for a `[ConditionalFact]` to actually want to use
Skip as it means unconditionally skip. This is generally only for short
term bug work arounds. Hence I changed `ConditionalFactAttribute` as
follows:

- Added a new Skip property that is obsolete and has no setter.
Completely prevents the code `[ConditionalFact(..., Skip = )`
- Added a Reason property documentation in the case the test is skipped.
- Added an AlwaysSkip property in the case you always want to skip.

* Add API bool Compilation.IsSymbolAccessibleWithin(
            ISymbol symbol,
            ISymbol within,
            ITypeSymbol throughType = null)
Also correct comments that incorrectly imply that inaccessible methods can be a member of a method group.
Fixes https://github.com/dotnet/roslyn/issues/24518
Fixes https://github.com/dotnet/roslyn/issues/23842