[clang] [NFC][clang] Add ubsan-trap-merge.c test to show absence of nomerge (PR #117649)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-x86_64-debian-dylib` running on `gribozavr4` while building `clang` at step 6 "test-build-unified-tree-check-clang".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/60/builds/13761

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-clang) failure: test (failure)
******************** TEST 'Clang :: CodeGen/ubsan-trap-merge.c' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 5: /b/1/llvm-x86_64-debian-dylib/build/bin/clang -cc1 -internal-isystem /b/1/llvm-x86_64-debian-dylib/build/lib/clang/20/include -nostdsysteminc -triple x86_64-linux-gnu -emit-llvm -fsanitize=signed-integer-overflow -fsanitize-trap=signed-integer-overflow -O3 -mllvm -ubsan-unique-traps /b/1/llvm-x86_64-debian-dylib/llvm-project/clang/test/CodeGen/ubsan-trap-merge.c -o -      | /b/1/llvm-x86_64-debian-dylib/build/bin/FileCheck /b/1/llvm-x86_64-debian-dylib/llvm-project/clang/test/CodeGen/ubsan-trap-merge.c
+ /b/1/llvm-x86_64-debian-dylib/build/bin/FileCheck /b/1/llvm-x86_64-debian-dylib/llvm-project/clang/test/CodeGen/ubsan-trap-merge.c
+ /b/1/llvm-x86_64-debian-dylib/build/bin/clang -cc1 -internal-isystem /b/1/llvm-x86_64-debian-dylib/build/lib/clang/20/include -nostdsysteminc -triple x86_64-linux-gnu -emit-llvm -fsanitize=signed-integer-overflow -fsanitize-trap=signed-integer-overflow -O3 -mllvm -ubsan-unique-traps /b/1/llvm-x86_64-debian-dylib/llvm-project/clang/test/CodeGen/ubsan-trap-merge.c -o -
/b/1/llvm-x86_64-debian-dylib/llvm-project/clang/test/CodeGen/ubsan-trap-merge.c:10:17: error: CHECK-LABEL: expected string not found in input
// CHECK-LABEL: define dso_local noundef i32 @f(
                ^
<stdin>:1:1: note: scanning from here
; ModuleID = '/b/1/llvm-x86_64-debian-dylib/llvm-project/clang/test/CodeGen/ubsan-trap-merge.c'
^
<stdin>:72:1: note: possible intended match here
define dso_local noundef i32 @m(i32 noundef %x, i32 noundef %y) local_unnamed_addr #0 {
^

Input file: <stdin>
Check file: /b/1/llvm-x86_64-debian-dylib/llvm-project/clang/test/CodeGen/ubsan-trap-merge.c

-dump-input=help explains the following input dump.

Input was:
<<<<<<
            1: ; ModuleID = '/b/1/llvm-x86_64-debian-dylib/llvm-project/clang/test/CodeGen/ubsan-trap-merge.c' 
label:10'0     X~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ error: no match found
            2: source_filename = "/b/1/llvm-x86_64-debian-dylib/llvm-project/clang/test/CodeGen/ubsan-trap-merge.c" 
label:10'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            3: target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128" 
label:10'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            4: target triple = "x86_64-unknown-linux-gnu" 
label:10'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            5:  
label:10'0     ~
            6: ; Function Attrs: nounwind 
label:10'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~
            .
            .
            .
           67:  %cond = tail call i32 @llvm.smin.i32(i32 %5, i32 %4) 
label:10'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           68:  ret i32 %cond 
label:10'0     ~~~~~~~~~~~~~~~
           69: } 
label:10'0     ~~
           70:  
label:10'0     ~
           71: ; Function Attrs: nounwind 
label:10'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~
...

```

</details>

https://github.com/llvm/llvm-project/pull/117649