Fix the build of libupc.

* upc-lib.h: Rename to clang-upc-lib.h
* lib/Frontend/InitPreprocessor.cpp: Remove predefines for exit, main.
  Pre-include clang-upc-lib.h for UPC language files regardless of
  optimization levels.
* runtime/libupc/smp/upc_alloc.upc:
* runtime/libupc/smp/upc_barrier.upc:
* runtime/libupc/smp/upc_lock.upc:
  Remove unnecessary include files.
* runtime/libupc/CMakeLists.txt: Create clang-upc-lib.h instead of upc-lib.h.
  Add IN_TARGET_LIB compiler option.