DebugNamesDWARFIndex: Implement GetFunctions method

Summary:
This patch implements the non-regex variant of GetFunctions. To share
more code with the Apple implementation, I've extracted the common
filtering code from that class into a utility function on the DWARFIndex
base class.

The new implementation also searching the accelerator table multiple
times -- previously it could happen that the apple table would return
the same die more than once if one specified multiple search flags in
name_type_mask. This way, I separate table iteration from filtering, and
so we can be sure each die is inserted at most once.

Reviewers: clayborg, JDevlieghere

Subscribers: aprantl, lldb-commits

Differential Revision: https://reviews.llvm.org/D47881

llvm-svn: 334273