[Orc][MCJIT][RuntimeDyld] Add symbol flags to symbols in RuntimeDyld. Thread the

[Orc][MCJIT][RuntimeDyld] Add symbol flags to symbols in RuntimeDyld. Thread the
new types through MCJIT and Orc.

In particular, add a 'weak' flag. When plumbed through RTDyldMemoryManager, this
will allow us to distinguish between weak and strong definitions and find the
right ones during symbol resolution.

llvm-svn: 231724