[ORC] Add a lookupFlags method to VSO.

lookupFlags returns a SymbolFlagsMap for the requested symbols, along with a
set containing the SymbolStringPtr for any symbol not found in the VSO.

The JITSymbolFlags for each symbol will have been stripped of its transient
JIT-state flags (i.e. NotMaterialized, Materializing).

Calling lookupFlags does not trigger symbol materialization.

llvm-svn: 323060