Enable ROOT to be built with prebuilt clang and llvm.

To do so one needs to pass -Dbuiltin_llvm=Off -Dbuiltin_clang=Off and the
PATH should contain the path to llvm-config.

Note this is not enabling ROOT to work with vanilla clang!

This patch allows ROOT to be built against a prebuilt clang and llvm from
https://root.cern.ch/git/{llvm.git,clang.git}. It allows to reduce ROOT's
build times (in cases when cmake decides to rebuild the in-tree llvm for
no good reason). It moves the common denominator of different ROOT builds
in one place to save space. It also allows easy switch between LLVM in
debug and release mode.

To build the external clang and llvm exactly in the same way as the
in-tree builds use:

CMAKE_FLAGS="\
            -DLLVM_ENABLE_WARNINGS=OFF                   \
            -DLLVM_INCLUDE_TESTS=OFF                     \
            -DCLANG_INCLUDE_TESTS=OFF                    \
            -DLLVM_INCLUDE_EXAMPLES=OFF                  \
            -DCLANG_BUILD_TOOLS=OFF                      \
            -DCLANG_TOOL_ARCMT_TEST_BUILD=OFF            \
            -DCLANG_TOOL_CLANG_CHECK_BUILD=OFF           \
            -DCLANG_TOOL_CLANG_FORMAT_BUILD=OFF          \
            -DCLANG_TOOL_CLANG_FORMAT_VS_BUILD=OFF       \
            -DCLANG_TOOL_CLANG_FUZZER_BUILD=OFF          \
            -DCLANG_TOOL_CLANG_IMPORT_TEST_BUILD=OFF     \
            -DCLANG_TOOL_CLANG_OFFLOAD_BUNDLER_BUILD=OFF \
            -DCLANG_TOOL_CLANG_RENAME_BUILD=OFF          \
            -DCLANG_TOOL_C_ARCMT_TEST_BUILD=OFF          \
            -DCLANG_TOOL_C_INDEX_TEST_BUILD=OFF          \
            -DCLANG_TOOL_DIAGTOOL_BUILD=OFF              \
            -DCLANG_TOOL_LIBCLANG_BUILD=OFF              \
            -DCLANG_TOOL_SCAN_BUILD_BUILD=OFF            \
            -DCLANG_TOOL_SCAN_VIEW_BUILD=OFF             \
            -DLLVM_BUILD_TOOLS=OFF                       \
            -DLLVM_TOOL_LLVM_AR_BUILD=OFF                \
            -DCLANG_TOOL_CLANG_OFFLOAD_BUNDLER_BUILD=OFF \
            -DLLVM_FORCE_USE_OLD_TOOLCHAIN=ON            \
            -DCLANG_ENABLE_STATIC_ANALYZER=OFF           \
            -DCLANG_ENABLE_ARCMT=OFF                     \
            -DCLANG_ENABLE_FORMAT=OFF                    \
            -DLLVM_TARGETS_TO_BUILD=host                 \
            -DLLVM_ABI_BREAKING_CHECKS=FORCE_OFF         \
            -DLLVM_ENABLE_ABI_BREAKING_CHECKS=OFF        \
            -DCMAKE_INSTALL_PREFIX=..                    \
            -DCMAKE_BUILD_TYPE=Debug"

cmake "$CMAKE_FLAGS"   ../../../sources/root-llvm/