Issue diagnostics when returning FP values on x86_64 without SSE1/2

Issue diagnostics when returning FP values on x86_64 without SSE1/2

Avoid using report_fatal_error, because it will ask the user to file a
bug. If the user attempts to disable SSE on x86_64 and them use floating
point, that's a bug in their code, not a bug in the compiler.

This is just a start. There are other ways to crash the backend in this
configuration, but they should be updated to follow this pattern.

Differential Revision: https://reviews.llvm.org/D27522

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@302835 91177308-0d34-0410-b5e6-96231b3b80d8