[lldb] Fix version string when using LLDB_REVISION but not LLDB_REPOSITORY

[lldb] Fix version string when using LLDB_REVISION but not LLDB_REPOSITORY

Summary:
lldb's format string (line one) is:
`lldb version $clang_version ($lldb_repo revision $lldb_revision)`

When only using $lldb_revision and not $lldb_repo, this might look like:
`lldb version 11 ( revision 12345)`
which looks pretty ugly.

Aside: I'm not sure we really need all the different versions since we've moved to the monorepo layout -- I don't think anyone is using different llvm/clang/lldb revisions, are they? We could likely tidy this up further if we knew how people consumed the output of lldb --version.

Reviewers: labath, JDevlieghere, friss

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D74859