Integrate libiomp CMake into LLVM CMake build system.

Integrate libiomp CMake into LLVM CMake build system.

This patch integrates the libiomp CMake build system into the LLVM CMake build
system so that users can checkout libiomp into the projects directory of llvm
and build llvm,clang, and libiomp all together.  These changes specifically
introduce a new install target which will put libraries and headers into the
correct locations when either a standalone build or part of llvm.
The copy_recipe() method has been removed in favor of the POST_BUILD method
to move headers into the exports subdirectory.  And lastly, the MicroTests.cmake
file was refactored which led to simpler target dependencies and a new target,
make libiomp-micro-tests, which performs the 5 small tests (test-relo,
test-touch, etc.) when called.

llvm-svn: 236534