[clang] Make explicitly-built modules independent of the CWD (llvm#164840)

PR llvm#150123 changed how we
normalize the modules cache path. Unfortunately, empty path would get
normalized to the current working directory. This means that even
explicitly-built PCMs that don't rely on the CWD now embed it, leading
to surprising behavior. This PR fixes that by normalizing an empty
modules cache path to an empty string.

Author: jansvoboda11

clang/lib/Frontend/CompilerInstance.cpp

@@ -563,14 +563,11 @@ void CompilerInstance::createPreprocessor(TranslationUnitKind TUKind) {
 std::string CompilerInstance::getSpecificModuleCachePath(StringRef ModuleHash) {
   assert(FileMgr && "Specific module cache path requires a FileManager");
 
-  if (getHeaderSearchOpts().ModuleCachePath.empty())
-    return "";
-
   // Set up the module path, including the hash for the module-creation options.
   SmallString<256> SpecificModuleCache;
   normalizeModuleCachePath(*FileMgr, getHeaderSearchOpts().ModuleCachePath,
                            SpecificModuleCache);
-  if (!getHeaderSearchOpts().DisableModuleHash)
+  if (!SpecificModuleCache.empty() && !getHeaderSearchOpts().DisableModuleHash)
     llvm::sys::path::append(SpecificModuleCache, ModuleHash);
   return std::string(SpecificModuleCache);
 }

clang/lib/Lex/HeaderSearch.cpp

@@ -2098,6 +2098,8 @@ std::string HeaderSearch::suggestPathToFileForDiagnostics(
 void clang::normalizeModuleCachePath(FileManager &FileMgr, StringRef Path,
                                      SmallVectorImpl<char> &NormalizedPath) {
   NormalizedPath.assign(Path.begin(), Path.end());
-  FileMgr.makeAbsolutePath(NormalizedPath);
-  llvm::sys::path::remove_dots(NormalizedPath);
+  if (!NormalizedPath.empty()) {
+    FileMgr.makeAbsolutePath(NormalizedPath);
+    llvm::sys::path::remove_dots(NormalizedPath);
+  }
 }

clang/test/Modules/explicit-build-cwd.c

@@ -0,0 +1,17 @@
+// This test checks that explicitly building the same module from different
+// working directories results in the same PCM contents.
+
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: mkdir %t/one
+// RUN: mkdir %t/two
+
+//--- module.modulemap
+module M { header "M.h" }
+
+//--- M.h
+
+// RUN: cd %t/one && %clang_cc1 -fmodules -emit-module %t/module.modulemap -fmodule-name=M -o %t/M_one.pcm
+// RUN: cd %t/two && %clang_cc1 -fmodules -emit-module %t/module.modulemap -fmodule-name=M -o %t/M_two.pcm
+
+// RUN: diff %t/M_one.pcm %t/M_two.pcm