[clang][modules] Ensure -nostdlib causes no manifest to be reported (llvm#161110)

When -nostdlib is specified, Clang should not report any
library‑provided module manifest, even if a manifest for the default
standard library is present.

Author: naveen-seth

clang/lib/Driver/Driver.cpp

@@ -6613,6 +6613,9 @@ std::string Driver::GetStdModuleManifestPath(const Compilation &C,
                                              const ToolChain &TC) const {
   std::string error = "<NOT PRESENT>";
 
+  if (C.getArgs().hasArg(options::OPT_nostdlib))
+    return error;
+
   switch (TC.GetCXXStdlibType(C.getArgs())) {
   case ToolChain::CST_Libcxx: {
     auto evaluate = [&](const char *library) -> std::optional<std::string> {

clang/test/Driver/modules-print-library-module-manifest-path.cpp

@@ -18,6 +18,14 @@
 // RUN:     --target=x86_64-linux-gnu 2>&1 \
 // RUN:   | FileCheck libcxx.cpp
 
+// check that -nostdlib causes no library-provided module manifest to
+// be reported, even when libc++.modules.json is present.
+// RUN: %clang -print-library-module-manifest-path \
+// RUN:     -nostdlib \
+// RUN:     -resource-dir=%t/Inputs/usr/lib/x86_64-linux-gnu \
+// RUN:     --target=x86_64-linux-gnu 2>&1 \
+// RUN:   | FileCheck libcxx-no-module-json.cpp
+
 // for macos there is a different directory structure
 // where the library and libc++.modules.json file are in lib
 // directly but headers are in clang/ver directory which