[ms-inline asm] Add support for parsing the '.' operator. Given,

[ms-inline asm] Add support for parsing the '.' operator.  Given,

[register].field

The operator returns the value at the location pointed to by register plus the
offset of field within its structure or union.  This patch only handles
immediate fields (i.e., [eax].4).  The original displacement has to be a
MCConstantExpr as well.
Part of rdar://12470415 and rdar://12470514

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@166632 91177308-0d34-0410-b5e6-96231b3b80d8