Optionally, Clang now builds using LLVM as an external library.

The user sets the variable CLANG_PATH_TO_LLVM_BUILD pointing to a LLVM
build directory or to a directory where LLVM was installed. When using
a non-installed LLVM build, it is necessary to set
CLANG_PATH_TO_LLVM_SOURCE as well.

llvm-svn: 124817