Fixes #7700

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.

Fixes #6109.

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.

Previously, the unix root "/" would not be considered a valid
path for pathmap to map to.  This commit fixes that, adds a
test case, and adds diagnostics for unparsable pathmaps.

Fixes #949

Fixes #4696.

- Undo some refactoring of CommonCompiler.ReadFileContent
- Remove LoadDirectiveResolver
- Make SourceReferenceResolver.ReadText detect encoding
- Make SourceReferenceResolver.ReadText Dispose its Stream
- Generate Diagnostic if source contains #load and SourceReferenceResolver
  is not specified in ParseOptions

These resourse strings reference an old namespace and type.  This caused me a bit of trouble when trying to find the Syntax class, as it also had changed to SyntaxFactory.

Add a new command line compiler switch "/reportanalyzer" to report analyzer execution times. Output is grouped by analyzer assemblies and is displayed in descending order of execution times.

NOTE: We do not display the total build time or the ratios between build time and analyzer execution time as the the actual wall clock time for analyzer execution is likely lesser due to multithreaded analyzer execution.

Fixes #2725

Add a new switch '/errorlog:' to VB and C# command line compilers to log all the compiler diagnostics into a log file.
Add command line parsing tests and error log content validation tests.

Expression Trees specifically require that element initializer methods are instance methods.
Fixes #310

Fixes #1029

While overall parameterless constructors in structs are valid from IL perspective, without a convenient way to declare them they were virtually nonexistent. As we performed more and more testing, we kept discovering cases where parameterless struct constructors caused inconsistent behavior in libraries or even in some versions of CLR.

After reconsidering the potential issues arising from breaking long standing assumptions, we decided it was best for our users to restore the requirement on struct constructors to always have formal parameters.