[cmake] Stop putting the revision info in LLVM_VERSION_STRING

[cmake] Stop putting the revision info in LLVM_VERSION_STRING

Summary:
This reduces the number of build actions after a no-op commit from
thousands to about six, which should be acceptable. If six actions is
still too many, developers can disable the LLVM_APPEND_VC_REV cmake
option.

llvm-config.h is a widely included header that should rarely change.
Before this patch, it would change after every re-configure. Very few
users of llvm-config.h need to know the precise version, and those that
do can migrate to incorporating LLVM_REVISION as provided by
llvm/Support/VCSRevision.h.

This should bring LLVM back to the behavior that it had before r306858
from June 30 2017. Most LLVM tools will now print a version string like
"6.0.0svn" instead of "6.0.0-git-c40c2a23de4".

Fixes PR34308

Reviewers: pcc, rafael, hans

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D37272

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312043 91177308-0d34-0410-b5e6-96231b3b80d8