[ORC] Improve debugging output for ORC.

(1) Print debugging output under a session lock to avoid garbled messages when
compiling on multiple threads.

(2) Name MaterializationUnits, add an ostream operator for them, and so they can
be easily referenced in debugging output, and have that ostream operator
optionally print code/data/hidden symbols provided by that materialization unit
based on command line options.

llvm-svn: 343323