[Sema][HLSL] Consolidate handling of HLSL attributes

This moves the sema checking of the entrypoint sensitive HLSL
attributes all into one place. This ended up being kind of large for a
couple of reasons:

- I had to move the call to CheckHLSLEntryPoint later in
  ActOnFunctionDeclarator so that we do this after redeclarations and
  have access to all of the attributes.

- We need to transfer the target shader stage onto the specified entry
  point before doing the checking.

- I removed "library" from the HLSLShader attribute value enum and
  just go through a string to convert from the triple - the other way
  was confusing and brittle.

Differential Revision: https://reviews.llvm.org/D158803