[clang] [ubsan] Add -fsanitize-merge (and -fno-sanitize-merge) (PR #120464)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

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

Full details are available at: https://lab.llvm.org/buildbot/#/builders/56/builds/14864

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

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

Command Output (stderr):
--
RUN: at line 1: /b/1/clang-x86_64-debian-fast/llvm.obj/bin/clang -cc1 -internal-isystem /b/1/clang-x86_64-debian-fast/llvm.obj/lib/clang/20/include -nostdsysteminc -emit-llvm -disable-llvm-passes -O -fsanitize=signed-integer-overflow -fsanitize-trap=signed-integer-overflow /b/1/clang-x86_64-debian-fast/llvm.src/clang/test/CodeGen/ubsan-trap-debugloc.c -o - -debug-info-kind=line-tables-only | /b/1/clang-x86_64-debian-fast/llvm.obj/bin/FileCheck /b/1/clang-x86_64-debian-fast/llvm.src/clang/test/CodeGen/ubsan-trap-debugloc.c
+ /b/1/clang-x86_64-debian-fast/llvm.obj/bin/clang -cc1 -internal-isystem /b/1/clang-x86_64-debian-fast/llvm.obj/lib/clang/20/include -nostdsysteminc -emit-llvm -disable-llvm-passes -O -fsanitize=signed-integer-overflow -fsanitize-trap=signed-integer-overflow /b/1/clang-x86_64-debian-fast/llvm.src/clang/test/CodeGen/ubsan-trap-debugloc.c -o - -debug-info-kind=line-tables-only
+ /b/1/clang-x86_64-debian-fast/llvm.obj/bin/FileCheck /b/1/clang-x86_64-debian-fast/llvm.src/clang/test/CodeGen/ubsan-trap-debugloc.c
/b/1/clang-x86_64-debian-fast/llvm.src/clang/test/CodeGen/ubsan-trap-debugloc.c:20:11: error: CHECK: expected string not found in input
// CHECK: [[LOC]] = !DILocation(line: 0
          ^
<stdin>:75:46: note: scanning from here
 call void @llvm.ubsantrap(i8 0) #4, !dbg !25, !nosanitize !8
                                             ^
<stdin>:75:46: note: with "LOC" equal to "!14"
 call void @llvm.ubsantrap(i8 0) #4, !dbg !25, !nosanitize !8
                                             ^
<stdin>:106:3: note: possible intended match here
!13 = !DILocation(line: 7, column: 7, scope: !5)
  ^

Input file: <stdin>
Check file: /b/1/clang-x86_64-debian-fast/llvm.src/clang/test/CodeGen/ubsan-trap-debugloc.c

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

Input was:
<<<<<<
            .
            .
            .
           70:  %8 = extractvalue { i32, i1 } %6, 1, !dbg !25, !nosanitize !8 
           71:  %9 = xor i1 %8, true, !dbg !25, !nosanitize !8 
           72:  br i1 %9, label %cont2, label %trap1, !dbg !25, !nosanitize !8 
           73:  
           74: trap1: ; preds = %cont 
           75:  call void @llvm.ubsantrap(i8 0) #4, !dbg !25, !nosanitize !8 
check:20'0                                                  X~~~~~~~~~~~~~~~~ error: no match found
check:20'1                                                                    with "LOC" equal to "!14"
           76:  unreachable, !dbg !25, !nosanitize !8 
check:20'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           77:  
check:20'0     ~
           78: cont2: ; preds = %cont 
check:20'0     ~~~~~~~~~~~~~~~~~~~~~~~
           79:  store volatile i32 %7, ptr %a.addr, align 4, !dbg !26, !tbaa !9 
check:20'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           80:  ret void, !dbg !27 
check:20'0     ~~~~~~~~~~~~~~~~~~~~
            .
...

```

</details>

https://github.com/llvm/llvm-project/pull/120464