PGO: Split out initialization of section bounds

Currently we register instrumentation data at runtime to determine the
bounds of the sections where the data lives.  Soon we'll implement
platform-specific linker magic to determine this at link time.

Move this logic to a separate file, so that our build system can choose
the correct platform-specific code.

No functionality change intended.

<rdar://problem/15943240>

llvm-svn: 204299