[ms] [llvm-ml] Add support for MASM STRUCT casting field accessors: (<TYPE> PTR <value>).<field>

Summary:
Add support for MASM STRUCT casting field accessors: (<TYPE> PTR <value>).<field>

Since these are operands, we add them to X86AsmParser. If/when we extend MASM support to other architectures (e.g., ARM), we will need similar changes there as well.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D83346