Implement the clang-side of detection for when to pass as

Implement the clang-side of detection for when to pass as
<2 x float> instead of double.  This works but can't be turned
on until I teach codegen to pass <2 x float> as one XMM register
instead of two.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@109790 91177308-0d34-0410-b5e6-96231b3b80d8