[Member name lookup] Eliminate non-lazy member loading.

Lazy member loading has been in use and the default for several years
now. However, the lazy loading was disabled for any type whose primary
definition was parsed even though some of its extensions could have
been deserialized, e.g., from a Clang module. Moreover, the non-lazy
path walked all of the extensions of such a type for all member name
lookup operations. Faced with a large number of extensions to the same
type (in my example, 6,000), this walk of the list of the extensions
could dominate type-checking time.

Eliminate all effects of the `-disable-named-lazy-member-loading`
flag, and always use the "lazy" path, which effectively does no work
for parsed type definitions and extensions thereof. The example with
6,000 extensions of a single type goes from type checking in 6 seconds
down to type checking in 0.6 seconds, and name lookup completely
disappears from the profiling trace.

The deleted tests relied on the flag that is now inert. They aren't by
themselves providing much value nowadays, and it's better to have the
simpler (and more efficient) implementation of member name lookup be
the only one.

lib/Frontend/CompilerInvocation.cpp

@@ -682,8 +682,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
                                               /*default*/ false);
   Opts.UseClangFunctionTypes |= Args.hasArg(OPT_use_clang_function_types);
 
-  Opts.NamedLazyMemberLoading &= !Args.hasArg(OPT_disable_named_lazy_member_loading);
-
   if (Args.hasArg(OPT_emit_fine_grained_dependency_sourcefile_dot_files))
     Opts.EmitFineGrainedDependencySourcefileDotFiles = true;