InstrProf: Reorganize files; no functionality change

Move functions around to prepare for some other changes.

  - Merge InstrProfilingExtras.h with InstrProfiling.h.  There's no
    benefit to having these split.

  - Rename InstrProfilingExtras.c to InstrProfilingFile.c.

  - Split actual buffer writing code out of InstrProfiling.c into
    InstrProfilingBuffer.c.

  - Drive-by corrections of a couple of header comments.

<rdar://problem/15943240>

llvm-svn: 204497