[Target] Move ObjCLanguageRuntime::LookupRuntimeSymbol into LanguageRuntime

Summary:
LookupRuntimeSymbol seems like a general LanguageRuntime method.
Although no other language runtime currently implements this, there's no
reason another language runtime couldn't use this.

Additionally, this breaks IRExecutionUnit's dependency on
ObjCLanguageRuntime.

Reviewers: compnerd, labath, JDevlieghere, davide

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D62795

llvm-svn: 362458