Decrease the verbosity of the -ftime-trace option

And move the relevant information in the doc.

Summary:
Currently, building a large software like Firefox shows
'Use chrome://tracing or Speedscope App (https://www.speedscope.app) for flamegraph visualization'
for each file.

Reviewers: anton-afanasyev

Reviewed By: anton-afanasyev

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68260

llvm-svn: 373308