Add special case bitcode support for DebugLoc. This avoids

having the bitcode writer materialize mdnodes for all the
debug location tuples when writing out the bc file and 
stores the information in a more compact form.  For example,
the -O0 -g bc file for combine.c in 176.gcc shrinks from
739392 to 512096 bytes.

This concludes my planned short-term debug info work.

llvm-svn: 100261