Use an unmanaged stream during PDB writing

Cross Platform build issues

More improvements to the ?. codegen.

Include pseudo-variables in CompileGetLocals

Invoke SetAdditionalFiles on compiler inputs when initializing from msbu...

Invoke SetAdditionalFiles on compiler inputs when initializing from msbuild csc task fails and we fall back to initializing from model. This should fix the local unit test failures in MSBuildWorkspace additional file unit tests that some developers are hitting.

Better handling of unary operators (like:  - foo?.Bar  )
Better lowering strategy for nullable receiver - we no longer rewrite to a ternary ?: in such cases which leads to more optimization opportunities.

Fix recursion problem in syntax normalizer

Show C# snippets with no SnippetType

Update ApplyChangesOperation doc comments

Trigger ThemeColorFixer on FormatMappingChanged

Introduce a CodePage1252Encoding class

Encoding.Default is not available on CoreCLR and neither is Encoding.GetEncoding(1252). So we have to roll our own.
Fixes #2081 

Fixes internal TFS bug #1156267. The behavior now matches that of Visual
Studio 2013.

Fixes a number of cross platform build issues:

- Managed tools which run as a part of build are now prefixed with
`mono ` in a standardized fashion.  This is now uniformly applied to
all tools for generating bound trees, syntax nodes, etc …
- Remove WPF references from TestUtiliites.csproj.
- Used reflection to access the GetRuntimeInterfaceAsObject API as it
is unavailable in the Mono APIs.

Script updates

Do not allow private members to be referred to in cref across assemblies

Do not crash when highlighting alias symbols defined via global imports.

Keep track of “bad” attribute applications to prevent PE generation in their presence.

Fixes #1904

Don't recommend #Const after #Const

Allow directive keyword recommenders to differentiate between the start
and middle of a preprocessor directive.

Fixes #722.

Treat System.Void as the absence of a type.

Fixes #1766.

Fixes #246

Extend Linux CI support

Couple of changes to the scripts:

- Factored out installing the PCL libraries to setup-pcl.sh.  Makes it
much easier to install PCL against custom builds of mono
- cibuild.sh accepts an optional `—os` option now. Makes it easier to
customize Jenkins based on the OS target.

Fixes #1123

Update the ThemeColorFixer to trigger on IClassificationFormatMap.ClassificationFormatMappingChanged. This allows us to update the tooltip classifications when the user manually changes colors or imports color settings.

We previously listened for WM_SYSCOLORCHANGED and WM_THEMECHANGED, but those are now redundant and have been removed.

In some scenarios, an attribute application can be “bad” due to an error type in referenced compilation, but without any diagnostics reported for the referencing compilation.

Added couple tests for ?. operator for cases missed in optimizations.

Test-only change

The tests arep assing, but codegen is not great.