[symbolizer] Support symbol+offset lookup
GNU addr2line supports lookup by symbol name in addition to the existing
address lookup. llvm-symbolizer starting from e144ae54dcb96838a6176fd9eef21028935ccd4f
supports lookup by symbol name. This change extends this lookup with
possibility to specify optional offset.
Now the address for which source information is searched for can be
specified with offset:
llvm-symbolize --obj=abc.so "SYMBOL func_22+0x12"
It decreases the gap in features of llvm-symbolizer and GNU addr2line.