InstrProf: Support profiling dlopen'd shared libraries

Shared objects are hard.  After this commit, we do the right thing when
profiling two separate shared objects that have been dlopen'd with
`RTLD_LOCAL`, when the main executable is *not* being profiled.

This mainly simplifies the writer logic.

  - At initialization, determine the output filename and truncate the
    file.  Depending on whether shared objects can see each other, this
    may happen multiple times.

  - At exit, each executable writes its own profile in append mode.

<rdar://problem/16918688>