AST: Move several Type::is...Type() functions to be inline.

 - These functions are both (a) very commonly called and (b) excellent
   candidates for CSE in the callers in which they are commonly used.
 - That isHalfType() is hot makes me sad, but it is trivially when inlined (and
   a huge waste of time when not!!!).
 - The extra IsEnumDeclComplete() function is a hack to break the cycle between
   Type.h and Decl.h, I'm not sure of how to do this more cleanly, but am open
   to ideas.

llvm-svn: 152126