Fixes #17971

Don't require a symbol at all for NavigateTo.

Move syntax checking of ObjectCreationExpressions out of the parser.

Pool more temporary values while doing camel case matching.

Fix analyzer exceptions when WorkspaceAnalyzerOptions not available

Move indexer parameter count checking out of the parser.

Move errors about params/__arglist being last out of the parser.

Move grammar checks for Anonymous-Type members out of the parser.

Reduce calls to AddError.

Remove special character handling in pattern matching.

* Removed AssemblySymbol.GetWellKnownType

Removed according to https://github.com/dotnet/roslyn/issues/18700

* Modified AssemblyAndNamespaceTests (WellKnownTypesAndAliases unit test)

Removed tests for the removed method AssemblySymbol.GetWellKnownType

Add Extract Interface Dialog tests

Fixes #16644

IVariableDeclaration refactor

Interface change of IHasArgumentsExpression, IInvocationExpression and IArgument 

The clearing of $LastExitCode in our Exec function is masking failures
of windows commands.  Cannot understand why this is the case but easy to
demonstrate in practice.

In cibuild-legacy.ps1 we cannot use Exec-Expression because it's a
bootstrapping problem.  Hence we have to use Invoke-Expression directly.

Even though we invoke a script via Invoke-Expression here we still must
check $LastExitCode.  In the case where the script exits explicitly with
an exit code this doesn't actually cause $? to be $false.  Hence we
still must check both values here.

Merge dev15.2.x to master

Match Roslyn EE to native EE behaviour with property accessibility

Improve parser error tolerance around parsing incomplete async members.

Added test to ensure that DebuggerBrowsable attribute does not affect accessibility on non-proxy types