Raise the logging level for 'after tracelet formation'

Summary: This stuff is largely the same as 'after initial translation', and seems
to me to be a little excessive for logging level 2. Raising it to 3.
(And changing some const to constexpr)

Reviewed By: @edwinsmith

Differential Revision: D1461826
Sanitize non-utf8 content in test output

Summary: Ran into this with a test which outputs cp1251 text
which json_encode (rightly) barfs on.  This results in the
test's diff output being reduced to null which is singularly
unhelpful in test output.

Use UConverter::transcode() to turn anything that isn't UTF-8
into the Unicode WTF character U+FFFD.

Hopefully we don't get too many of these kinds of tests.

Reviewed By: @ptarjan

Differential Revision: D1462059
Turn off JIT::Timer by default in OSS build

Summary: In the absence of our special __vdso_clock_gettime_ns syscall, it
falls back to getrusage. The former is fast enough that there's no reason to
not have it on by default but the same isn't true of the latter.

Reviewed By: @jdelong

Differential Revision: D1461996