AST: extend MS decoration handling for extended vectors

We correctly handled extended vectors of non-floating point types.
However, we have the Intel style builtins which MSVC also supports which
do overlap in sizes with the floating point extended vectors.  This
would result in overloading of floating point extended vector types
which matched sizes (e.g. <3 x float> would be backed by a <4 x float>
and thus match sizes) to be mangled similarly.  Extended vectors are a
clang extension which live outside of the builtins, so mangle them all
similarly.  This change just extends the current scheme to treat
floating point types similar to the way that we treat other types
currently.

This now allows the swift runtime to be built for Windows again.

llvm-svn: 345479