CodeGen: make target builtins support languages

This extends the target builtin support to allow language specific annotations
(i.e. LANGBUILTIN).  This is to allow MSVC compatibility whilst retaining the
ability to have EABI targets use a __builtin_ prefix.  This is merely to allow
uniformity in the EABI case where the unprefixed name is provided as an alias in
the header.

llvm-svn: 212196