PGO: Splitting implementation files; no functionality change

Split implementation files along a uses-libc/shouldn't-use-libc
boundary.

  - InstrProfiling.h is a shared header.

  - InstrProfiling.c provides an API to extract profiling data from the
    runtime, but avoids the use of libc.  Currently this is a lie:
    __llvm_pgo_write_buffer() uses `FILE*` and related functions.  It
    will be updated soon to write to a `char*` buffer instead.

  - InstrProfilingExtras.c provides a more convenient API for
    interfacing with the profiling runtime, but has logic that does (and
    will continue to) use libc.

<rdar://problem/15943240>

llvm-svn: 204268