We want the dwarf AT_producer for assembly source files to match clang's
AT_producer.  Which includes clang's version information so we can tell
which version of the compiler was used.

This is the first of two steps to allow us to do that.  This is the llvm-mc
change to provide a method to set the AT_producer string.  The second step,
coming soon to a clang near you, will have the clang driver pass the value
of getClangFullVersion() via an flag when invoking the integrated assembler
on assembly source files.

rdar://12955296