Factored some function-like type reasoning out of SemaDeclAttr and onto Decl itself. This allows for more declarative subjects in attribute tablegen where the attribute appertains to something function-like, but not strictly a FunctionDecl.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199387 91177308-0d34-0410-b5e6-96231b3b80d8