Reapply and patch "[CIR][ABI] Apply CC lowering pass by default (#842)"

[sitio-couto]

This reverts commit 8f699fd and fixes some issues, namely:

• CC lowering pass will no longer fail if the function has no AST information that won't be used.
• Fixed CC lowering not disabling when running certain cc1 compilation commands.
• CC lowering can now be disabled when calling cir-opt and cir-translate.
• Compilation commands that generate Object files should now invoke CC lowering by default.

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[sitio-couto]

@bcardosolopes (friendly ping)

[ChuanqiXu9]

I found this patch caused a regression on vastart. Reproducer:

// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-llvm %s -o %t.ll
// RUN: FileCheck --input-file=%t.ll %s -check-prefix=LLVM-X86

#include <stdarg.h>
void f1(int n, ...) {
  va_list valist;
  va_start(valist, n);
  va_end(valist);
}

[sitio-couto]

Hey @ChuanqiXu9 !

I found this patch caused a regression on vastart

I think the regression is due to the variadic f1 function. The CC lowering pass doesn't handle variadics yet, but it is also not asserting against them. I'll add an assertion to clarify this. In any case, the -fno-clangir-call-conv-lowering should "fix" it.

[ChuanqiXu9]

Hey @ChuanqiXu9 !

I found this patch caused a regression on vastart

I think the regression is due to the variadic f1 function. The CC lowering pass doesn't handle variadics yet, but it is also not asserting against them. I'll add an assertion to clarify this. In any case, the -fno-clangir-call-conv-lowering should "fix" it.

Got it. Thanks. Would you like to create an issue that? And we can track it more explicitly.

[ChuanqiXu9]

I sent #974 to track this more formally