DebugNamesDWARFIndex: Add ability to lookup variables

Summary:
This patch adds the ability to lookup variables to the DWARF v5 index
class.

During review we discovered an inconsistency between how the existing
two indexes handle looking up qualified names of the variables:
- manual index would return a value if the input string exactly matched
  the demangled name of some variable.
- apple index ignored the context and returned any variable with the
  same base name.

So, this patch also rectifies that situation:
- it removes all context handling from the index classes. The
  GetGlobalVariables functions now just take a base name. For manual
  index, this meant we can stop putting demangled names into the
  variable index (this matches the behavior for functions).
- context extraction is put into SymbolFileDWARF, so that it is common
  to all indexes.
- additional filtering based on the context is also done in
  SymbolFileDWARF. This is done via a simple substring search, which is
  not ideal, but it matches what we are doing for functions (cf.
  Module::LookupInfo::Prune).

Reviewers: clayborg, JDevlieghere

Subscribers: aprantl, lldb-commits

Differential Revision: https://reviews.llvm.org/D47781

llvm-svn: 334181