[Tooling] Make clang-tool find libc++ dir on mac when running on a file without compilation database.

Summary:
This is a regression of r348365.

When clang-tools run on a file without a complation database (`clang-check /tmp/t.cc`),
we will use fixed compilation database as a fallback. However the actual compiler
path in the fallback complation command is just `clang-tool` which is
insufficient to detect the libc++ dir.

Reviewers: ilya-biryukov, EricWF

Reviewed By: ilya-biryukov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D56680

1 files changed, 16 insertions, 0 deletions

diff --git a/clang/test/Tooling/clang-check-mac-libcxx-fixed-compilation-db.cpp b/clang/test/Tooling/clang-check-mac-libcxx-fixed-compilation-db.cpp
new file mode 100644
index 00000000000..b072380c04e
--- /dev/null
+++ b/clang/test/Tooling/clang-check-mac-libcxx-fixed-compilation-db.cpp
@@ -0,0 +1,16 @@
+// Clang on MacOS can find libc++ living beside the installed compiler.
+// This test makes sure our libTooling-based tools emulate this properly with
+// fixed compilation database.
+//
+// RUN: rm -rf %t
+// RUN: mkdir %t
+//
+// Install the mock libc++ (simulates the libc++ directory structure).
+// RUN: cp -r %S/Inputs/mock-libcxx %t/
+//
+// RUN: cp clang-check %t/mock-libcxx/bin/
+// RUN: cp "%s" "%t/test.cpp"
+// RUN: %t/mock-libcxx/bin/clang-check -p "%t" "%t/test.cpp" -- -stdlib=libc++
+
+#include <mock_vector>
+vector v;