[ExecutionEngine] FindFunctionNamed: Skip declarations

Summary:
Basically all other methods that look up functions by name skip them if they are mere declarations.
Do the same in FindFunctionNamed.

Reviewers: lhames

Reviewed By: lhames

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7068

llvm-svn: 227227