[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

llvm/include/llvm/MC/MCParser/MCAsmParser.h

@@ -170,6 +170,11 @@ class MCAsmParser {
 
   virtual bool isParsingMasm() const { return false; }
 
+  virtual bool LookUpFieldOffset(StringRef Base, StringRef Member,
+                                 unsigned &Offset) {
+    return true;
+  }
+
   /// Parse MS-style inline assembly.
   virtual bool parseMSInlineAsm(
       void *AsmLoc, std::string &AsmString, unsigned &NumOutputs,

llvm/include/llvm/MC/MCParser/MCTargetAsmParser.h

@@ -334,7 +334,7 @@ class MCTargetAsmParser : public MCAsmParserExtension {
 
   /// SemaCallback - The Sema callback implementation.  Must be set when parsing
   /// ms-style inline assembly.
-  MCAsmParserSemaCallback *SemaCallback;
+  MCAsmParserSemaCallback *SemaCallback = nullptr;
 
   /// Set of options which affects instrumentation of inline assembly.
   MCTargetOptions MCOptions;