…ork/lazy-method-loading

Note, this is a no-op when the LLDB setting
`plugin.typesystem.clang.experimental-redecl-completion` is not set (which is currently the default). So this patch has no effect unless the user explicitly opts into it.

The type-completion rework (aka redecl-completion) implemented in apple#8222 comes with a large performance penalty, since we now eagerly complete `RecordType`s. Completing a `RecordType` previously unconditionally resolved all member functions of said type. With redecl-completion completion, however, this meant we were now pulling in many more definitions than needed. Without redecl-completion, this isn't a problem, since importing method parameters is cheap (they are imported minimally), so we wouldn't notice that we always resolved all member functions.

This patch tries to load methods lazily when in redecl-completion mode. We do this by introducing a new `ExternalASTSource::FindExternalVisibleMethods` API which Clang calls when parsing a member access expression. The callback into LLDB will do a `FindFunctions` call for all methods with the method name we're looking for, filters out any mismatches, and lets Clang continue with its parsing. We still load following methods eagerly:
1. virtual functions: currently overrides are resolved in `CompleteRecordType`
2. operators: currently I couldn't find a point at which Clang can call into LLDB here to facilitate lazy loading
3. ctors/dtors: same reason as (2)

In our benchmark harness, we saw this patch bring down redecl-completion expression evaluation performance on-par with top-of-tree expression evaluation.