Add script 'UpdateCMakeLists.pl'. This script can be used (manually) to update

Add script 'UpdateCMakeLists.pl'. This script can be used (manually) to update
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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@75833 91177308-0d34-0410-b5e6-96231b3b80d8