[CMake] Move -stdlib=libc++ handling into its own file.

r200744 moved this into cmake/config-ix.cmake, so that it would happen very
early in the build process. However, standalone builds of Clang and other
external projects never include this file (which is correct).

Now, -stdlib=libc++ and the LLVM_COMPILER_IS_GCC_COMPATIBLE option are
both set in a new include file, HandleLLVMStdlib, which is included by
both config-ix.cmake and HandleLLVMOptions.cmake. This preserves existing
behavior for projects relying on HandleLLVMOptions and still does the
right thing for builds of LLVM itself.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200811 91177308-0d34-0410-b5e6-96231b3b80d8
[CMake] Deliberately get all LLVM library dependencies for standalone builds.

CMake won't expand the dependency graph for us if the dependencies are in
another project, which leads to link errors in the standalone build.
This is a refinement of r200765.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200812 91177308-0d34-0410-b5e6-96231b3b80d8