[ORC] Add findSymbolIn() wrapper to C bindings, take #2.

Re-appply r333147, reverted in r333152 due to a pre-existing bug. As
D47308 has been merged in r333206, the OSX issue should now be
resolved.

In many cases JIT users will know in which module a symbol
resides. Avoiding to search other modules can be more efficient. It
also allows to handle duplicate symbol names between modules.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D44889

llvm-svn: 333215