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>

llvm-svn: 204381