CodeGen: Expand creal and cimag into complex field loads

PR 14529 was opened because neither Clang or LLVM was expanding
calls to creal* or cimag* into instructions that just load the
respective complex field.  After some discussion, it was not
considered realistic to do this in LLVM because of the platform
specific way complex types are expanded.  Thus a way to solve
this in Clang was pursued.  GCC does a similar expansion.

This patch adds the feature to Clang by making the creal* and
cimag* functions library builtins and modifying the builtin code
generator to look for the new builtin types.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@170455 91177308-0d34-0410-b5e6-96231b3b80d8