[clang] Fix use of funified-lto and save-temps flags together (PR #162763)

[Alexey Bader via cfe-commits]

https://github.com/bader created https://github.com/llvm/llvm-project/pull/162763

> clang -flto -funified-lto -save-temps test.c

Fails with the following error message:

```bash
module flag identifiers must be unique (or of 'require' type)
!"UnifiedLTO"
fatal error: error in backend: Broken module found, compilation aborted!
clang: error: clang frontend command failed with exit code 70 (use -v to see invocation)
```

Here is what the driver does when `-save-temps` flag is set:

> clang -flto -funified-lto -save-temps test.c -ccc-print-phases
>          +- 0: input, "test.c", c
>       +- 1: preprocessor, {0}, cpp-output
>    +- 2: compiler, {1}, ir
> +- 3: backend, {2}, lto-bc
> 4: linker, {3}, image

The IR output of "compiler" step has "UnifiedLTO" module flag. "backend"
step adds another module flag with "UnifiedLTO" identifier, which
invalidates the LLVM IR module.

>From 1db7dc4d96deb01e7a03b4a9068a005789da1352 Mon Sep 17 00:00:00 2001
From: Alexey Bader <alexey.bader at intel.com>
Date: Thu, 9 Oct 2025 19:41:04 -0700
Subject: [PATCH] Fix use of funified-lto and save-temps flags together

> clang -flto -funified-lto -save-temps test.c

Fails with the following error message:

```bash
module flag identifiers must be unique (or of 'require' type)
!"UnifiedLTO"
fatal error: error in backend: Broken module found, compilation aborted!
clang: error: clang frontend command failed with exit code 70 (use -v to see invocation)
```

Here is what the driver does when `-save-temps` flag is set:

> clang -flto -funified-lto -save-temps test.c -ccc-print-phases
>          +- 0: input, "test.c", c
>       +- 1: preprocessor, {0}, cpp-output
>    +- 2: compiler, {1}, ir
> +- 3: backend, {2}, lto-bc
> 4: linker, {3}, image

The IR output of "compiler" step has "UnifiedLTO" module flag. "backend"
step adds another module flag with "UnifiedLTO" identifier, which
invalidates the LLVM IR module.
---
 clang/lib/CodeGen/BackendUtil.cpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
index 2d959827d6972..d28d886b3551e 100644
--- a/clang/lib/CodeGen/BackendUtil.cpp
+++ b/clang/lib/CodeGen/BackendUtil.cpp
@@ -1199,7 +1199,8 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
       }
     }
 
-    if (shouldEmitUnifiedLTOModueFlag())
+    if (shouldEmitUnifiedLTOModueFlag() &&
+        !TheModule->getModuleFlag("UnifiedLTO"))
       TheModule->addModuleFlag(llvm::Module::Error, "UnifiedLTO", uint32_t(1));
   }