Reduce C# emit test allocations

preserve encoding over temporary storage

some code clean up based on PR feedback

This change reduces the VM used and memory allocated in the emit
tests by moving the WinRT tests to their own DLL and removing
the extra mscorlib reference.

The cause of most of the allocations in the Emit unit tests was
serializing the WinRT assemblies across app domains. There were
three reasons the DLLs needed to be serialized:

1) PeVerify currently requires a separate app domain to be used if
   any of the references of the assembly to be verified have already
   been loaded into the current app domain and have a different MVID.

2) The WinRT tests all use a separate version of mscorlib, so they
   will all reference an assembly with a different MVID than the one
   loaded.

3) Most of the WinRT tests were accidentally pulling two references
   to mscorlib, also always leading to a conflict.

All of these issues should be resolved. The CSharp.Emit unit tests
now allocated 400 MB less and have > 100 MB smaller peak VM.

Fixes #386.

Code review feedback for PR #488

Fix #251: Preview Changes dialog always shows additional files as empty

Fixing the assembly name of BasicSystemRuntimeAnalyzers project which ha...

Fixing the assembly name of BasicSystemRuntimeAnalyzers project which had the same name as another project (copy\paste error)

Fix Code Model bug reported by @carlos-j-quintero

Unify VS Setup

Lazy doc comments for signature help

Made the retrieval and parsing of XML documentation comments for signature help items lazy. This saves allocations and CPU time for operations such as typing within methods like Console.WriteLine (which has 19 overloads)

Also, cache the result of the last XML fragment parse (since we often ask to parse the same fragment over and over)

Add regression test for Issue #62

Fix malformed XML doc comments

Fixed a NullReferenceException introduced by the last commit (range
validation)

Use compact property getter syntax.
Added range checking to Signature's constructor.

Fix doc comment typo.