Fixes https://devdiv.visualstudio.com/DevDiv/_workitems?id=233669.

- VB: Report a warning about AssemblyKeyFile and AssemblyKeyName attributes being ignored by public signing.
- Disallow public signing for Net modules. The fact that the signing is public is not preserved in a module. The fact of signing is preserved by emitting attributes that are going to be ignored anyway if public signing is used.

Closes #11427.

Removing diagnostic IDs from previous versions of the VB compiler that are no longer generated by Roslyn.

Fixes #14895

This is the same fix as was implemented for C# in #13806, but for VB.

* tuples with names differing only by case are not allowed
* names that match predefined element names except for case do not result in additional names
* name-related errors are checked in case-insensitive fasion

* Implement source link feature

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

* Reduce code duplication across VB and C#

Fixes #8287.

Improves comments around the IsCompleteSubmission.
Throws argument exception when the syntax is not a submission.
Uses the Parser.CheckFeatureAvailability to check multiline strings.
Renames the IsScriptOrInteractive to IsScript.
Handle incomplete members such as an annotation.
Do not show completions when inside of a directive.
Add tests for symbol completion provider.

I've done some manual testing, but there are no unit tests.

Existing unit tests are pass, except for ParseXml.Bug674567.  The fix is
either to change the baseline or to merge
https://github.com/dotnet/roslyn/pull/3957.

Conflicts:
	src/Compilers/VisualBasic/Portable/Scanner/Scanner.vb
	src/Compilers/VisualBasic/Portable/Syntax/CompilationUnitSyntax.vb
	src/Compilers/VisualBasic/Test/Semantic/Compilation/CompilationAPITests.vb

Sometimes called "fake sign" or "OSS sign" public signing is including
the public key in an output assembly and setting the "signed" flag, but
not actually signing the assembly with a private key. This is useful for
open source projects where people want to build assemblies which are
compatible with the released "fully signed" assemblies, but don't have
access to the private key used to sign the assemblies. Since almost no
consumers actually need to check if the assembly is fully signed, these
publicly built assemblies are useable in almost every scenario that the
fully signed one would be used in.

This PR implements support only for C# -- VB will be added soon. If
being used at the command line, the /publicsign flag can be passed to
csc and the /keyfile flag can specify the public key. Unlike fully
signing, a full key pair encoded in the SNK file format is not currently
supported. When using /publicsign, just the public key must be in the
/keyfile file.

When using the API, the public key can be passed directly using the
CryptoPublicKey CompilationOption.