Makes the symbol lookup return no symbols when completing `base.` inside
of a REPL.

Include HasValue check in BoundSwitchStatement so any display class allocation is moved outside of the check

The flag name is "IOperation". Register opeartion actions and calling
SemanticModel.GetOperation while the feature is disabled will throw an
InvalidOperationException. Also log a non-fatal-watson when
SemanticModel.GetOperation crashes.

This reverts commit 7bb0b4ea.

Log non-fatal-watson if exception is not caused by stack overflow in OperationWalker.

Replace exception-throwing with return value `Invalid` when unexpected value is encountered inIOperation APIs

Changing the names of the parameters to `isStatic` and `isInstance` to be consistent with the naming guidelines of boolean parameters.

This reverts commit d43d4780.

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.

Also fixes a crash in `BoundConversion.ConversionKind` in C#.

Also fixes crash in `ArgumentBase.IsInvalid` property when code has error.

1) Remove the field Default - analyzer authors can avoid calling the ConfigureCodeAnalysis API if they want driver's default behavior.

2) Change the value of ReportDiagnostics to 2

Fixes #9048

Replaced all references of `IReferenceExpression` with `IOperation`.

Fixes #7477

Since it could be `IInvalidExpression` in case of error.

Doing so will cause crashes if such line is obtained and is also a compat break.

Fixes #8200

Fixes #7700

Prevent DeriveArgument from throwing ArrayIndexOutOfRangeException when presented with semantically malformed bound trees.

and use that to diagnose cases where the compiler produces
compilation events later than it should.