[Expression parser] Look up module symbols before hunting globally

When it resolves symbol-only variables, the expression parser
currently looks only in the global module list. It should prefer
the current module.

I've fixed that behavior by making it search the current module
first, and only search globally if it finds nothing. I've also
added a test case.

After review, I moved the core of the lookup algorithm into
SymbolContext for use by other code that needs it.

Thanks to Greg Clayton and Pavel Labath for their help.

Differential Revision: https://reviews.llvm.org/D33083

llvm-svn: 303223