The only supported calling convention for X86-64 uses

SSE, so we can't return floating point values if this
is disabled.  Detect this error for clang.

With SSE1 only, f64 is a problem; it can be done, but
neither llvm-gcc nor clang has ever generated correct
code for it.  Since nobody noticed this I think it's
OK to treat it as an error for now.

This also handles SSE-sized vectors of floating point.
8207686, 8204109.

llvm-svn: 109201