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