[ms] [llvm-ml] Add initial MASM STRUCT/UNION support

Summary:
Add support for user-defined types to MasmParser, including initialization and field access.

Known issues:
- Omitted entry initializers (e.g., <,0>) do not work consistently for nested structs/arrays.
- Size checking/inference for values with known types is not yet implemented.
- Some ml64.exe syntaxes for accessing STRUCT fields are not recognized.
  - `[<register>.<struct name>].<field>`
  - `[<register>[<struct name>.<field>]]`
  - `(<struct name> PTR [<register>]).<field>`
  - `[<variable>.<struct name>].<field>`
  - `(<struct name> PTR <variable>).<field>`

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D75306