libc++: Install headers in include/, rather than lib/

When using the configure build system, the libc++ headers were being
installed in lib/, whereas cmake installs them in include/. Since
include/ makes more sense for headers, we'll make both systems install
headers there.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194833 91177308-0d34-0410-b5e6-96231b3b80d8
Darwin: Look for libc++ headers in include/, rather than lib/

Up until now we were expecting that when libc++ is installed alongside
clang the headers would be in lib/, which was true if the configure
build was used and false if the cmake build was.

We've now corrected the configure build to install in include/, and
with this change we'll be able to find the correct headers with both
build systems.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194834 91177308-0d34-0410-b5e6-96231b3b80d8
Remove an unused local from r194827

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194835 91177308-0d34-0410-b5e6-96231b3b80d8
clang-cl: Make the driver parse all(?) msvc flags

Even if we don't support a flag, we should be able to parse it
to provide a better error message than the current default
"error: no such file or directory: '/foo'" (which we should probably
also tweak, btw).

This also tries to clean up the test file a bit.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194837 91177308-0d34-0410-b5e6-96231b3b80d8