[ASan] Allow the users of SymbolizationLoop to make use of the --dsym_hint...

[ASan] Allow the users of SymbolizationLoop to make use of the --dsym_hint option in llvm-symbolizer

Let the users of SymbolizationLoop define a function that produces the list of .dSYM hints (possible path to the .dSYM bundle) for the given binary.
Because the hints can't be added to an existing llvm-symbolizer process, we spawn a new symbolizer process ones each time a new hint appears.
Those can only appear for binaries that we haven't seen before.

llvm-svn: 222535