Ignore the documentation-suggested location for compile_commands.json

According to http://clang.llvm.org/docs/HowToSetupToolingForLLVM.html
it's suggested that compile_commands.json in the root of the LLVM source tree
should be a symlink to the json file produced by your build system of choice.

So here's a patch so it doesn't turn up in git status, etc.

llvm-svn: 162305