That fixes issues when using directories (source or build) containing e.g. spaces.
It *should* not break anything as it's just adding quotes where they were missing.
FYI, cxx was already correctly quoted. I added quoting to gdb. There are other issues in other parts of LLVM tho (e.g. one cannot build clang in a build dir with spaces) but I'll handle that in different patches.
I believe that this doesn't work properly on Windows. The quoting rules on Windows are quite different. Using " instead of ' would work. However, depending on how this is used, CMake should be able to handle the quoting appropriately per platform.
Please use pipes.quote for the argument quoting.
Please use pipes.quote for the quoted parameters (and the same throughout the rest of the lit configuration).