the list of files in the CMakeLists.txt files.

It works by scanning (starting from the working directory from which it is run)
all subdirectories containing CMakeLists.txt files. It then updates the list of
C and C++ files in the 'add_llvm_library' and 'add_clang_library' lists. Sources
file lists are emitted in lexical order.

llvm-svn: 75833