Perform Clang IRGen during check (#6569)

Background:
https://docs.google.com/document/d/1wi85FRiWh4X9A-gCYMVGKR40-q5fM6-3JaSpePk-XCY/edit?usp=sharing
And specifically this work is essentially an alternative to #5543

Clang's code generation is implemented through an ASTListener
(clang::CodeGenerator) that is attached throughout Clang's
parsing/sema/code
generation phases and acts on Clang AST incrementally throughout that
process.

Prior to this patch, Carbon has only created the CodeGenerator during
Carbon's
`lower` phase, missing out on key callbacks that would be made by Clang
during
`check`. Some of these issues were addressed by #6237 and #6483 - but
there were
still remaining cases where the delayed processing lead to missing
functionality.

With #6483 much of the Clang code that made multithreaded complexity of
#5543 is
no longer present, and we have access to the point of ASTListener
registration
so we can register the CodeGenerator there and consume its resulting
llvm::Module during lower.

Examples of some of the bugs this addresses are seen in the linked doc,
and
checked in as tests in this change in
`clang_code_generator_callbacks.carbon`

An indicental bug that's also fixed, and caused all the other test case
churn,
is that the `CodeGenerator` created during `lower` wasn't getting passed
the
Clang `CodeGenOpts` and was creating its own default - so, most notably,
optimization flags were not respected. This meant that the LLVM IR from
Clang
was always -O0 style IR (optnone, no inlinehint, no TBAA, etc). With
this
change, now the Clang IRGen gets the real `CodeGenOpts` and respects
optimization/other flags specified there.

This is only meant to be a rough proof of concept - I'm totally open to
reworking this in any way (even quite substantially) if folks have ideas
about
how this should be implemented most generally/elegantly/etc.

toolchain/check/check.cpp

@@ -485,7 +485,8 @@ auto CheckParseTrees(
   for (int check_index = 0;
        check_index < static_cast<int>(ready_to_check.size()); ++check_index) {
     auto* unit_info = ready_to_check[check_index];
-    CheckUnit(unit_info, &tree_and_subtrees_getters, fs, clang_invocation,
+    CheckUnit(unit_info, &tree_and_subtrees_getters, fs,
+              unit_info->unit->llvm_context, clang_invocation,
               options.vlog_stream)
         .Run();
     for (auto* incoming_import : unit_info->incoming_imports) {
@@ -534,7 +535,8 @@ auto CheckParseTrees(
     // incomplete imports.
     for (auto& unit_info : unit_infos) {
       if (unit_info.imports_remaining > 0) {
-        CheckUnit(&unit_info, &tree_and_subtrees_getters, fs, clang_invocation,
+        CheckUnit(&unit_info, &tree_and_subtrees_getters, fs,
+                  unit_info.unit->llvm_context, clang_invocation,
                   options.vlog_stream)
             .Run();
       }