Update LLVM to a recent commit (#6599)

This brings some fixes:
- The handling of `zlib` and `zstd` are much cleaner
- Three of our patches are no longer needed

This also includes the fixes from #6562

It also moves us from `zlib` to `zlib-ng` which is a much better basis
for what we want, and likely makes our toolchain faster when generating
debug info at least.

It fixes another API change in terms of which headers provide the
`createInvocation` we use.

Lastly, it cleans up the deps test to correctly recognize the wrappers
for `zlib-ng` and `zstd`, as well as improving the documentation for why
we allow dependencies on them.

toolchain/check/cpp/import.cpp

@@ -1307,7 +1307,7 @@ static auto CreateFunctionSignatureInsts(Context& context, SemIR::LocId loc_id,
     -> std::optional<FunctionSignatureInsts> {
   context.full_pattern_stack().PushFullPattern(
       FullPatternStack::Kind::ImplicitParamList);
-  std::optional pop = llvm::make_scope_exit(
+  std::optional pop = llvm::scope_exit(
       [&context] { context.full_pattern_stack().PopFullPattern(); });
   auto implicit_param_patterns_id =
       MakeImplicitParamPatternsBlockId(context, loc_id, *clang_decl);