[CMake] Bug 14109 - CMake build for compiler-rt should use just-built clang

Summary:
Many small improvements to LLVM_BUILD_EXTERNAL_COMPILER_RT.

* Works correctly with Ninja by working around CMake Bug 14771 (https://cmake.org/Bug/view.php?id=14771)
* Has install-compiler-rt target, and installs as part of the default install target
* Sets the install paths properly so that it matches the non-standalone build
* Only generate the test targets if(LLVM_INCLUDE_TESTS)

Reviewers: samsonov, Bigcheese

Differential Revision: http://reviews.llvm.org/D13399

llvm-svn: 250064