Fix the calling convention of Mingw64 long double values

GCC uses the x87DoubleExtended model for long doubles, and passes them
indirectly by address through function calls.

Also replace the existing mingw-long-double assembly emitting test with
an IR-level test.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@251567 91177308-0d34-0410-b5e6-96231b3b80d8