[llvm] [BOLT] Fix LLVM_APPEND_VC_REV support (PR #142410)

[Paschalis Mpeis via llvm-commits]

paschalis-mpeis wrote:

Hey folks,

Let's wait for @aaupov to clear up the confusion.
Not sure about @h-vetinari, but I believe @kwk  the rest were tagged as they have originally reviewed:
- #97130

That PR brought some extra CMake configuration but inadvertently broke the `LLVM_APPEND_VC_REV` flag.
This patch fixes that.

https://github.com/llvm/llvm-project/pull/142410