[flang] Do not print backtrace for most TODOs.

There are two ways to interrupt flang when a TODO is hit: abort or
exit with non-zero exit code. Abort implies printing the backtrace
and "PLEASE submit a bug report" message. We used to use abort
only in debug compiler build. Since the TODOs are already known
problems it may be counterproductive to ask debug compiler users
to submit bug reports. In addition, the TODO messages are pretty
self-explanatory, so the backtrace printing seems to be redundant.

This change makes all TODOs non-aborting. I added TODO_.*TRACE macros
for TODOs that may benefit from the backtrace and the "bug report"
message in the debug compiler. These macros are currently unused.

Differential Revision: https://reviews.llvm.org/D143761