PGO: Add explicit static initialization

Instead of relying on explicit static initialization from translation
units, create a new file, InstrProfilingRuntime.cc, with an
__llvm_pgo_runtime variable.  After this commit (and its pair in clang),
the driver will create a use of this variable.  Unless the user defines
their own version, the new object file will get pulled in, including
that C++ static initialization that calls
__llvm_pgo_register_write_atexit.

The result is that, at least on Darwin, static initialization typically
consists of a single function call, which registers a writeout functino
atexit.  Furthermore, users can skip even this behaviour by defining
their own __llvm_pgo_runtime.

<rdar://problem/15943240>
PGO: Update interface for writing instrumentation data to file

__llvm_pgo_write_default_file() was a bad name, since it checked the
environment (it wasn't just a default file).

  - Change __llvm_pgo_write_file() to __llvm_pgo_write_file_with_name()
    and make it static.

  - Rename __llvm_pgo_write_default_file() to __llvm_pgo_write_file().

  - Add __llvm_pgo_set_filename(), which sets the filename for
    subsequent calls to __llvm_pgo_write_file().

<rdar://problem/15943240>