[msan] Compile the libatomic.c test with a C compiler

[arichardson]

Summary

Otherwise we end up compiling in C++ mode and on FreeBSD
/usr/include/stdatomic.h is not compatible with C++ since it uses _Bool.

[guiand]

I think we can actually just drop the #include <stdatomic.h> line here. It was used for a previous version of the test but I forgot to remove it after some changes.

[eugenis]

Does it help to replace %clangxx_msan with %clang_msan instead?

[arichardson]

In D84510#2172577, @eugenis wrote:

Does it help to replace %clangxx_msan with %clang_msan instead?

That sounds like the better solution. I wasn't aware that that substitution also existed.

[arichardson]

use %clang_msan

[emaste]

LGTM

[guiand]

LGTM

compiler-rt/test/msan/libatomic.c

-// RUN: %clangxx_msan -fsanitize-memory-track-origins=2 -latomic -DTEST_STORE -O0 %s -o %t && %run %t 2>&1
-// RUN: %clangxx_msan -fsanitize-memory-track-origins=0 -latomic -DTEST_LOAD -O0 %s -o %t && not %run %t 2>&1 | FileCheck %s --check-prefix=CHECK
-// RUN: %clangxx_msan -fsanitize-memory-track-origins=2 -latomic -DTEST_LOAD -O0 %s -o %t && not %run %t 2>&1 | FileCheck %s --check-prefix=CHECK --check-prefix=CHECK-SHADOW
+// RUN: %clangxx_msan -xc -fsanitize-memory-track-origins=2 -latomic -DTEST_STORE -O0 %s -o %t && %run %t 2>&1
+// RUN: %clangxx_msan -xc -fsanitize-memory-track-origins=0 -latomic -DTEST_LOAD -O0 %s -o %t && not %run %t 2>&1 | FileCheck %s --check-prefix=CHECK
+// RUN: %clangxx_msan -xc -fsanitize-memory-track-origins=2 -latomic -DTEST_LOAD -O0 %s -o %t && not %run %t 2>&1 | FileCheck %s --check-prefix=CHECK --check-prefix=CHECK-SHADOW
 
 #include <sanitizer/msan_interface.h>
 #include <stdatomic.h>
 
 typedef struct __attribute((packed)) {
   uint8_t val[3];
 } i24;