[Stats] Add ALWAYS_ENABLED_STATISTIC enabled regardless of LLVM_ENABLE_STATS.
The intended usage is to measure relatively expensive operations. So the
cost of the statistic is negligible compared to the cost of a measured
operation and can be enabled all the time without impairing the
compilation time.
rdar://problem/
55715134
Reviewers: dsanders, bogner, rtereshin
Reviewed By: dsanders
Subscribers: hiraditya, jkorous, dexonsmith, ributzka, cfe-commits, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D68252
llvm-svn: 374490