[gn build] Add a script checking if sources in BUILD.gn and CMakeLists.txt files match.

Also fix a missing file in lib/Support/BUILD.gn found by the script.

The script is very stupid and assumes that CMakeLists.txt follow the standard
LLVM CMakeLists.txt formatting with one cpp source file per line. Despite its
simplicity, it works well in practice.

It would be nice if it also checked deps and maybe automatically applied its
suggestions.

Differential Revision: https://reviews.llvm.org/D54930

llvm-svn: 347925