Add a location to MS inline asm blobs

This isn't nearly as elaborate as the GCC inline asm which emits an
array of source locations, but it's very, very hard to trigger backend
diagnostics in MS inline asm because we parse it up front with good
source information, unlike GCC inline asm.

Currently I can trigger a "inline assembly requires more registers than
available" diagnostic with this code:

  void foo();
  void bar() {
    __asm pusha
    __asm call foo
    __asm popa
  }

However, if I committed that as a test case, I would have to remove it
once I fix PR20052.