[llvm] [TableGen] Detect invalid -D arguments and fail. (PR #102813)

[Rahul Joshi via llvm-commits]

================
@@ -54,9 +74,17 @@ TGLexer::TGLexer(SourceMgr &SM, ArrayRef<std::string> Macros) : SrcMgr(SM) {
   PrepIncludeStack.push_back(
       std::make_unique<std::vector<PreprocessorControlDesc>>());
 
-  // Put all macros defined in the command line into the DefinedMacros set.
-  for (const std::string &MacroName : Macros)
+  // Add all macros defined on the command line to the DefinedMacros set.
+  // Check invalid macro names and print fatal error if we find one.
+  for (const std::string &MacroName : Macros) {
+    const char *Begin = MacroName.c_str();
+    const char *End = lexMacroName(Begin);
----------------
jurahul wrote:

I don't think that is true. For example, the std::string that is passed into the Macro is constructed using StringRef::str() in CommandLine.h here:

```
  // Return true on error.
  bool parse(Option &, StringRef, StringRef Arg, std::string &Value) {
    Value = Arg.str();
    return false;
  }
```

so I don't think we can assume its null terminated. Which means that the common function I added will need to accept a StringRef, and the call from prepLexMacroName() has to pass in the appropriate length (of the string if it knows, or std::string::npos to retain the current behavior. Let me make that change, so that code functionally identical to the existing one. 

https://github.com/llvm/llvm-project/pull/102813