[ms-inline asm] Avoid a false positive assertion

[ms-inline asm] Avoid a false positive assertion

Assertion failed: (Start.isValid() == End.isValid() && "Start and end should
either both be valid or both be invalid!")

when parsing inline asm.  SMLoc assumes that the first char * in the source is
invalid.  However, when parsing an inline asm the mnemonic is at this location.
I don't want to change SMLoc, so use a trivial workaround.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162381 91177308-0d34-0410-b5e6-96231b3b80d8