* use io exception message

* disable Culture test

* run tests on english ui culture

* use hresult detection code that works cross-language

* fix indentation

* Only run FileShareDeleteCompatibility_Xplat on an english machine

* Brace on new line

* Harden compiler against exceptions during stream dispose

It's possible (likely even) for one of the streams the compiler uses for
output files will throw during disposal. Currently this crashes the
compiler. This change adds a wrapper type around stream to catch
exceptions during disposal and report them as diagnostics instead.

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems?id=217719

* Add VB tests

* Respond to PR comments

* Respond to PR comments

* Respond to PR comments

* Conform to style guidelines

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems?id=217718.

Fixes #12550.

Fixes #7588

This change drops support for .NET 4.5 and retargets to desktop
4.6 + CoreCLR. It also fixes up various analyzer warnings that
were created due to the move.

Fixes #12926

and SpecifiedLanguageVersion to the parse options.

* Implement source link feature

* Fix CommonCompiler to open source link stream, add E2E tests

* Reduce code duplication across VB and C#

supported language version when used in APIs.
Also update the C# 7 feature status doc to bring it up-to-date.

We also change the default language version for VB from 14 to 15, and
put the binary literals and digit separators under language version 15.

This PR takes the approach of immediately transforming a "Latest"
language version to the actual latest language version. Thus, the extra enum value
should never be persisted as part of an options object, but only transiently
appear as an argument to an API call setting or resetting the language version.

The native compiler allowed some command line arguments to have quotes around their values:

```
csc /pdb:full h.cs
csc /pdb:"full" h.cs
```

Roslyn broke this behavior post 1.0 during our general cleanup of command line parsing.  This is one case that was missed.

This change fixes the C# side of the problem.  Doing it separately to unblock customers.  Will follow up with VB later.

Commit which introduced the regression:

https://github.com/dotnet/roslyn/commit/258d4aa10ab7de73fb6eba7a2f9b8772597d0470#diff-97a7a5298fc5036af649f59ff021c791

Tracking issue #12427

Handle an empty quotation in the compiler switch.
Add csc and vbc help information.
Make the description of the option generic.

Add further tests for the instrument switch.
Unquote quoted strings passed to the switch.
Make the WithInstrument return this if the value it sets hasn't changed.
Add comments for the Instrument fields.

Conflicts:
	src/Compilers/Core/CodeAnalysisTest/Emit/EmitOptionsTests.cs

Right now there are two code paths for signing using a keyfile:

1) Full & delay signing implemented using a strong-name provider, which
   calls into the CLR when necessary to do signing.
2) Public signing, which extracts the key directly from the file and
   embeds it manually into the assembly using only portable code.

The tricky part is that (1) also implements a bunch of legacy semantics
on how to resolve the key file from various search paths, since the key
file can also be specified via an attribute.

This behavior is already deprecated, so rather than implement the
complicated file resolution for public sign as well, this PR changes the
command line compilers to always produce an absolute path for the key
file before passing it to CompilationOptions. CreateCompilation now also
has a check to enforce that CryptoKeyFile is an absolute path when
PublicSign is set.

Fixes #8360.

Ignore dlls that come from a 'packages' directory when searching for viable metadata references to search.

Handled all WorkItem instances that refer to migrated work items.

Corrects a couple of items noted in #7677