harden both DetectTypeCircularity and GetDependenceChain

This reverts commit 7debfc47.

This reverts commit a03d8b8e.

VB: Always import instance fields for an Enum type in order to be able to infer underlying type of its elements.

Address https://devdiv.visualstudio.com/DefaultCollection/DevDiv/_workitems?_a=edit&id=193333.

This reverts commit d43d4780.

This reverts commit d43d4780.

This reverts commit 7bb0b4ea.

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.

Scripting scenarios set a special flag in the ReferenceManager to enable
additional tie-breaking rules in the presence of conflicting references.
Enable that same flag for the EE, which also routinely encounters
conflicting references.

Fixes internal bug 161294.

Fixes #8088.

Strengthen implementation of MetadataHelpers.GetInfoForImmediateNamespaceMembers against unconventional/obfuscated namespace names in metadata.

Fixes #7396.

Adjust implementation of NamedTypeSymbol equality to treat generic type, constructed with modified its own type parameters, equal  to its definition when custom modifiers should be ignored.

Fixes #6372.

VB didn't check if publicsign was set before reporting an error
about the keyfile only containing a public key. This also checks
if public sign is set before reporting said error.