MS inline asm: Fix null SMLoc when 'ptr' is missing after dword & co

This improves the diagnostics from the regular assembler, but more
importantly it fixes an assertion when parsing inline assembly.  Test
landing in Clang.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214468 91177308-0d34-0410-b5e6-96231b3b80d8