Remove some hard-coded specialness for thread-safety attributes from the...

Remove some hard-coded specialness for thread-safety attributes from the parser, and made it more declarative. If an attribute is allowed to appear on a function definition when late parsed, it can now use the FunctionDefinition attribute subject. It's treated as a FunctionDecl for most purposes, except it also gets exposed on the AttributeList so that it can be used while parsing.

llvm-svn: 199676