Time profiler: small fixes and optimizations

Summary: Fixes from Roman's review here: https://reviews.llvm.org/D58675#1465336

Reviewers: lebedev.ri

Subscribers: hiraditya, mgrang, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D60663

llvm-svn: 358448