[Orc] Fix common symbol support in ORC.

Common symbol support in ORC was broken in r270716 when the symbol resolution
rules in RuntimeDyld were changed. With the switch to lazily materialized
symbols in r277386, common symbols can be supported by having
RuntimeDyld::emitCommonSymbols search for (but not materialize!) definitions
elsewhere in the logical dylib.

This patch adds the 'Common' flag to JITSymbolFlags, and the necessary check
to RuntimeDyld::emitCommonSymbols.

llvm-svn: 277397