PGO: Remove explicit static initialization

Remove the remaining explicit static initialization from translation
units, at least on Darwin.  Instead, create a use of __llvm_pgo_runtime,
which will pull in required code from compiler-rt.

After this commit (and its pair in compiler-rt), a user can define their
own __llvm_pgo_runtime to satisfy this undefined symbol and call the
functions in compiler-rt directly.

<rdar://problem/15943240>