[clang-doc] concatenate SymbolIDs to truncated mangled names (#159490)
Previously, if mangled names were too long to be used as filenames, the
object's SymbolID was used as a filename. This worked for length
restrictions, but made URLs/filenames inconsistent. This patch truncates
the mangled name and appends the SymbolID. Thus, we can keep some
context in the URL/filename while preserving uniqueness.