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 affect unless the user explicitly opts into it.

The type-completion rework (aka redecl-completion) implemented in
swiftlang#8222 comes with
a large performance penalty, since we now eagerly complete
`RecordType`s. Completing a `RecordType` previously unconditionally
resolved all member function 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 parses 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 it's 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 on-par with top-of-tree expression evaluation.