[cxx-interop] Avoid "libstdc++ not found" warning when -nostdinc++ is passed

When explicitly asked not to load the C++ standard library, Swift should not emit warnings for missing libstdc++.

This fixes a compiler warning when building the Cxx module on Linux.

Author: egorzhdan

lib/ClangImporter/ClangIncludePaths.cpp

@@ -288,6 +288,12 @@ static void getLibStdCxxFileMapping(
                                               stdlibArgStrings);
   auto parsedStdlibArgs = parseClangDriverArgs(clangDriver, stdlibArgStrings);
 
+  // If we were explicitly asked to not bring in the C++ stdlib, bail.
+  if (parsedStdlibArgs.hasArg(clang::driver::options::OPT_nostdinc,
+                              clang::driver::options::OPT_nostdincxx,
+                              clang::driver::options::OPT_nostdlibinc))
+    return;
+
   Path cxxStdlibDir;
   if (auto dir = findFirstIncludeDir(parsedStdlibArgs,
                                      {"cstdlib", "string", "vector"}, vfs)) {

test/Interop/Cxx/stdlib/Inputs/module.modulemap

@@ -69,3 +69,9 @@ module StdFunction {
   requires cplusplus
   export *
 }
+
+module NoCXXStdlib {
+  header "no-cxx-stdlib.h"
+  requires cplusplus
+  export *
+}

test/Interop/Cxx/stdlib/Inputs/no-cxx-stdlib.h

@@ -0,0 +1,5 @@
+// This file intentionally does not import anything from the C++ stdlib.
+
+inline int my_sum(int a, int b) {
+  return a + b;
+}

test/Interop/Cxx/stdlib/no-cxx-stdlib.swift

@@ -0,0 +1,5 @@
+// RUN: %target-typecheck-verify-swift -I %S/Inputs -cxx-interoperability-mode=upcoming-swift -Xcc -nostdinc++
+
+import NoCXXStdlib
+
+let _ = my_sum(12, 15)