[ms] [llvm-ml] Use default RIP-relative addressing for x64 MASM.

Summary:
When parsing 64-bit MASM, treat memory operands with unspecified base register as RIP-based.

Documented in several places, including https://software.intel.com/en-us/articles/introduction-to-x64-assembly: "Unfortunately, MASM does not allow this form of opcode, but other assemblers like FASM and YASM do. Instead, MASM embeds RIP-relative addressing implicitly."

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D73227