Revert "Define LLVM_CXX explicitly as CMake variable to make it work"

This reverts commit 999a62e6ad29090e74f615bd054048d3bf58d91d.
Will be put into build rules for CIFactory instead.
Revert "This CMake variable might even exist."

This reverts commit 9a3b191ce196fb11cfce0a872c7d08283acb9240.
Not needed anymore.
Revert "Remember build-time CXX in llvm-config.h."

This reverts commit 667f0400d0091f96e3d1d7efda5e03275dd5078a.

Put into CIFactory rules instead.
Generate cling/lib/Interpreter/cling-compiledata.h.
Generate cling-compiledata.h (CMake).
Don't change llvm-config.h for cling; use our own. Add CXXFLAGS.

Fixes the stdlib detection e.g. on MacOS 10.8 with libc++:
before, the compiler got invoked without the CXXFLAGS and thus without
-std=c++11 nor especially -stdlib=libc++, making it impossible to determine the
correct stdlib includes paths.