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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162305 91177308-0d34-0410-b5e6-96231b3b80d8

.gitignore

@@ -27,6 +27,7 @@ cscope.files
 cscope.out
 autoconf/aclocal.m4
 autoconf/autom4te.cache
+compile_commands.json
 
 #==============================================================================#
 # Directories to ignore (do not add trailing '/'s, they skip symlinks).