Add support for line profiling. Very work-in-progress.

Use debug info in the IR to find the directory/file:line:col. Each time that location changes, bump a counter.

Unlike the existing profiling system, we don't try to look at argv[], and thusly don't require main() to be present in the IR. This matches GCC's technique where you specify the profiling flag when producing each .o file.

The runtime library is minimal, currently just calling printf at program shutdown time. The API is designed to make it possible to emit GCOV data later on.

llvm-svn: 129340