[CMake] Avoid __FakeVCSRevision.h with no git repository
    
    Set the return variable to "" in find_first_existing_vc_file to
    say that there is a repository, but no file to depend on. This works
    transparently for all other callers that handle undefinedness and
    equality to an empty string the same way.
    
    Use the knowledge to avoid depending on __FakeVCSRevision.h if there
    is no git repository at all (for example when building a release) as
    there is no point in regenerating an empty VCSRevision.h.
    
    Differential Revision: https://reviews.llvm.org/D92718
    
    (cherry picked from commit 6e890ec7beb0874464a0af9f84e41a987f968b23)