[clang-repl] Teach clang-repl how to load PCHs (reprise) (#157359)

This is an updated version of @vgvassilev's PR from last year here:
https://github.com/llvm/llvm-project/pull/94166

In short, it includes:

1. The fix for a blocking issue where `clang::Interpreter` (and thus
`clang-repl`) cannot resolve symbols defined in a PCH
2. A test to prove this is working
3. A new hidden flag for `clang-repl` so that `llvm-lit` can match the
host JIT triple between the PCH and `clang-repl`; previously, they may
differ in some cases
4. Everything based on the latest LLVM main

Shout out to @kylc for finding a logic issue which had us stumped for a
while (and securing the
[bounty](https://github.com/jank-lang/jank/issues/446)).

---------

Co-authored-by: Vassil Vassilev <v.g.vassilev@gmail.com>
Co-authored-by: Kyle Cesare <kcesare@gmail.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/ModuleBuilder.cpp b/clang/lib/CodeGen/ModuleBuilder.cpp
index 8c1fee8..96f3f62 100644
--- a/clang/lib/CodeGen/ModuleBuilder.cpp
+++ b/clang/lib/CodeGen/ModuleBuilder.cpp
@@ -138,6 +138,8 @@ namespace {
       assert(!M && "Replacing existing Module?");
       M.reset(new llvm::Module(ExpandModuleName(ModuleName, CodeGenOpts), C));
 
+      IRGenFinished = false;
+
       std::unique_ptr<CodeGenModule> OldBuilder = std::move(Builder);
 
       Initialize(*Ctx);
@@ -179,6 +181,10 @@ namespace {
     }
 
     bool HandleTopLevelDecl(DeclGroupRef DG) override {
+      // Ignore interesting decls from the AST reader after IRGen is finished.
+      if (IRGenFinished)
+        return true; // We can't CodeGen more but pass to other consumers.
+
       // FIXME: Why not return false and abort parsing?
       if (Diags.hasUnrecoverableErrorOccurred())
         return true;
@@ -292,8 +298,9 @@ namespace {
         if (Builder)
           Builder->clear();
         M.reset();
-        return;
       }
+
+      IRGenFinished = true;
     }
 
     void AssignInheritanceModel(CXXRecordDecl *RD) override {