[clang][modules] Only compute affecting module maps with implicit search (llvm#87849)

When writing out a PCM, we compute the set of module maps that did
affect the compilation and we strip the rest to make the output
independent of them. The most common way to read a module map that is
not affecting is with implicit module map search. The other option is to
pass a bunch of unnecessary `-fmodule-map-file=<path>` arguments on the
command-line, in which case the client should probably not give those to
Clang anyway.

This makes serialization of explicit modules faster, mostly due to
reduced file system traffic.

(cherry picked from commit 51786eb)

Author: jansvoboda11

clang/lib/Serialization/ASTWriter.cpp

@@ -161,8 +161,13 @@ static TypeCode getTypeCodeForTypeClass(Type::TypeClass id) {
 
 namespace {
 
-std::set<const FileEntry *> GetAffectingModuleMaps(const Preprocessor &PP,
-                                                   Module *RootModule) {
+std::optional<std::set<const FileEntry *>>
+GetAffectingModuleMaps(const Preprocessor &PP, Module *RootModule) {
+  // Without implicit module map search, there's no good reason to know about
+  // any module maps that are not affecting.
+  if (!PP.getHeaderSearchInfo().getHeaderSearchOpts().ImplicitModuleMaps)
+    return std::nullopt;
+
   SmallVector<const Module *> ModulesToProcess{RootModule};
 
   const HeaderSearch &HS = PP.getHeaderSearchInfo();
@@ -4682,8 +4687,16 @@ void ASTWriter::computeNonAffectingInputFiles() {
     if (!Cache->OrigEntry)
       continue;
 
-    if (!isModuleMap(File.getFileCharacteristic()) ||
-        llvm::is_contained(AffectingModuleMaps, *Cache->OrigEntry))
+    // Don't prune anything other than module maps.
+    if (!isModuleMap(File.getFileCharacteristic()))
+      continue;
+
+    // Don't prune module maps if all are guaranteed to be affecting.
+    if (!AffectingModuleMaps)
+      continue;
+
+    // Don't prune module maps that are affecting.
+    if (llvm::is_contained(*AffectingModuleMaps, *Cache->OrigEntry))
       continue;
 
     IsSLocAffecting[I] = false;