InstrProf: Fix shared object profiling

Change the API of the instrumented profiling library to work with shared
objects.

  - Most things are now declared hidden, so that each executable gets
    its own copy.

  - Initialization hooks up a linked list of writers.

  - The raw format with shared objects that are profiled consists of a
    concatenated series of profiles.  llvm-profdata knows how to deal
    with that since r208938.

<rdar://problem/16918688>