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

Add CodeStyle option to allow users to customer the 'Invoke Delegate With Conditional Access' analyzer.

This reverts commit 4a4dbfae, reversing
changes made to c751c769.

Update OOP analyzers to calculate OpenFileOnly based on options

Only Error- and Warning-level options will run on closed files.
Suggestions will only run on open files.

added OpenFileOnly in IBuiltInAnalyzer and removed RunInProcess

now, all builtin analyzer that is not set to only run on open files will run in OOP.

any builtin analyzer that can't run in OOP should mark it as open file only.

for analyzers that return only hidden severity but return different severity on runtime should use the new API to control whether it want it to run full solution or not. if set to run on full solution, it will run in OOP.

Now that feature names aren't used for serialization anymore, we can
just get rid of the constants and use the containing type name.
Also, update more options to use nameof where appropriate.

Fixes #12714

SimplifyTypeName, QualifyMemberAccess, and
CSharpUse[Implicit/Explicit]TypeDiagnosticAnalyzer were declaring one
DiagnosticDescriptor per DiagnosticSeverity level in SupportedDiagnostics.
After this change, they only initially report a DiagnosticSeverity.Hidden
descriptor, which prevents them from running on closed files.

There are longer-term plans to allow these to run on closed files but
under closer control of the user.