[Release-testers] RFC: Update LLVM_VERSION_SUFFIX CMake variable for release candidates

[Tom Stellard via Release-testers]

Hi,

I would like to propose that we start using the LLVM_VERSION_SUFFIX
CMake variable for release candidates.  For example:
after the release/13.x branch is created, instead of changing
LLVM_VERSION_SUFFIX from "git" to "", we would change it to "rc1",
then after the 13.0.0-rc1 release is tagged, we would update the
variable to "rc2", etc. Then right before the final release has been
tagged, we would set it to ""

The library SONAME's currently include LLVM_VERSION_SUFFIX, so this
change will cause each release candidate to have a different SONAME
for libraries.  This is correct for X.Y.0 releases, since it's possible
for a library's ABI to change between release candidates.  However,
for X.Y.1 releases, we do not want to modify the SONAME's at all, so
the build system will need to be updated to accommodate this change.

Ensuring that the SONAME's change when the library ABI's change is
the main motivating reason for proposing this change.  However,
I think in general it is also better for the project to be able to
distinguish between release candidates and final releases.

Note that clang does not currently include LLVM_VERSION_SUFFIX in its
--version output, and I'm not proposing change this as part of this
RFC.

- Tom