[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