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

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@351222 91177308-0d34-0410-b5e6-96231b3b80d8