Support looking up absolute symbols

The Swift stdlib uses absolute symbols in the dylib to communicate
feature flags to the process. LLDB's expression evaluator needs to be
able to find them. This wires up absolute symbols so they show up in
the symtab lookup command, which is also all that's needed for them to
be visible to the expression evaluator JIT.

rdar://85093828

Differential Revision: https://reviews.llvm.org/D113445