Make clang/test/Index/pch-from-libclang.c pass in more places

- fixes the test on macOS with LLVM_ENABLE_PIC=OFF
- together with D57343, gets the test to pass on Windows
- makes it run everywhere (it seems to just pass on Linux)

The main change is to pull out the resource directory computation into a
function shared by all 3 places that do it. In CIndexer.cpp, this now works no
matter if libclang is in lib/ or bin/ or statically linked to a binary in bin/.


Differential Revision: https://reviews.llvm.org/D57345

llvm-svn: 352803