A PDL match result pattern doesn't have a debug name, which is what
"debug-only=greedy-rewriter" prints out during rewrite. So it's hard to find
out which PDL pattern is being applied from the debug log.
This commit names the generated rewriter in PDLInterp with the optional
attribute "sym_name" of PDL PatternOp. Then the name of a PDL pattern can be
set as debug name of its match result PDLByteCodePattern.