[clang] Pass -clear-ast-before-backend in Clang::ConstructJob()

This clears the memory used for the Clang AST before we run LLVM passes.

https://llvm-compile-time-tracker.com/compare.php?from=d0a5f61c4f6fccec87fd5207e3fcd9502dd59854&to=b7437fee79e04464dd968e1a29185495f3590481&stat=max-rss
shows significant memory savings with no slowdown (in fact -O0 slightly speeds up).

For more background, see
https://lists.llvm.org/pipermail/cfe-dev/2021-September/068930.html.

Turn this off for the interpreter since it does codegen multiple times.

Differential Revision: https://reviews.llvm.org/D111270

Author: aeubanks

clang/lib/Driver/ToolChains/Clang.cpp

@@ -4660,6 +4660,7 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   // cleanup.
   if (!C.isForDiagnostics())
     CmdArgs.push_back("-disable-free");
+  CmdArgs.push_back("-clear-ast-before-backend");
 
 #ifdef NDEBUG
   const bool IsAssertBuild = false;

clang/lib/Interpreter/Interpreter.cpp

@@ -113,6 +113,10 @@ CreateCI(const llvm::opt::ArgStringList &Argv) {
 
   Clang->getTarget().adjust(Clang->getDiagnostics(), Clang->getLangOpts());
 
+  // Don't clear the AST before backend codegen since we do codegen multiple
+  // times, reusing the same AST.
+  Clang->getCodeGenOpts().ClearASTBeforeBackend = false;
+
   return std::move(Clang);
 }