[Umbrella] ThroughMLIR path status

[wenhu1024]

Issue Description

Encountered an error when using ClangIR through the MLIR to LLVMIR path. The direct path from CIR to LLVMIR works correctly, but the path through MLIR fails. Is there a plan in the ClangIR project to support the complete pathway from CIR through MLIR to LLVM IR for generating executable files?

Steps to Reproduce

1. Using this simple code:

#include <stdio.h>

int main(){
    printf("hello\n");
    return 0;
}

2. Using CIR direct to LLVMIR path (works correctly):

../bin/clang -fclangir -fclangir-direct-lowering hello.c -o hello

Execution result:

./hello
hello

3. Using CIR through MLIR to LLVMIR path (fails):

../bin/clang -fclangir -fno-clangir-direct-lowering hello.c -o hello

Error message:

loc(fused["/usr/include/stdio.h":363:1, "/usr/include/stdio.h":363:56]): error: 'func.func' op symbol declaration cannot have public visibility
fatal error: error in backend: The pass manager failed to lower CIR to LLVMIR dialect!
clang: error: clang frontend command failed with exit code 70 (use -v to see invocation)

[bcardosolopes]

Different folks have been involved with paving that path, mostly recently @keryell, if you want to work on that y'all can sync on that.

[bcardosolopes]

Actually, you might want to follow #1219, closing this one