[Stats] Convert some ad-hoc header search stats to ALWAYS_ENABLED_STATISTIC.

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: 374581