--Implement GetRuntimeLookupSignatureNameForClass/GetRuntimeLookupSignatureNameForMethod in bridge code

--GetRuntimeLookupSignatureNameForMethod is used for all usg lookups, regardless of whether method is generic/nongeneric.  In the case of a nongeneric method, I do lookup using TypeDesc instead of MethodDesc.

--Call these from jitinterface instead of the stub code.

--Made method NativeLayoutSignature public so its callable from the bridge

--Made class NativeLayoutSignatureNode public as method NativeLayoutSignature is now public

--Merged method signature and type signature hashtables into a single hashtable GlobalNameLookUpSignatureMap
----This lets us avoid creating multiple SSRs for identical types
----This is a possible perf hit since we're indexing on char* now instead of handles - there are a couple options in the future to mitigate this

[tfs-changeset: 1653636]