[clang-tidy] Added -system-headers option.

Added -system-headers option to allow display of warnings from system headers.
This is needed for testing libcxx, for example.


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@220826 91177308-0d34-0410-b5e6-96231b3b80d8
[clang-tidy] Updated docs for r220826.


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@220830 91177308-0d34-0410-b5e6-96231b3b80d8