[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

llvm-svn: 166632