[clang] [ASTWriter] Do not allocate source location space for module maps used only for textual headers (PR #116374)

[Boaz Brickner via cfe-commits]

================
@@ -184,14 +184,30 @@ GetAffectingModuleMaps(const Preprocessor &PP, Module *RootModule) {
   const SourceManager &SM = PP.getSourceManager();
   const ModuleMap &MM = HS.getModuleMap();
 
-  llvm::DenseSet<FileID> ModuleMaps;
-
-  llvm::DenseSet<const Module *> ProcessedModules;
-  auto CollectModuleMapsForHierarchy = [&](const Module *M) {
+  // Module maps used only by textual headers are special. Their FileID is
+  // non-affecting, but their FileEntry is (i.e. must be written as InputFile).
+  enum AffectedReason : bool {
+    ARTextualHeader = 0,
+    ARImportOrTextualHeader = 1,
+  };
+  auto AssignMostImportant = [](AffectedReason &L, AffectedReason R) {
+    L = std::max(L, R);
+  };
----------------
bricknerb wrote:

Seems like using `LHS` and `RHS` is more common (and more readable IMO) in Clang than `L` and `R`.
```suggestion
  auto AssignMostImportant = [](AffectedReason &LHS, AffectedReason RHS) {
    LHS = std::max(LHS, RHS);
  };
```

https://github.com/llvm/llvm-project/pull/116374