[compiler-rt] [test] [ubsan] Build .cpp files with %clangxx

[mstorsjo]

Summary

This fixes linker errors in mingw configurations, where the ubsan
runtime needs the C++ standard library when instrumenting C++ code.

[phosek]

LGTM

compiler-rt/test/ubsan/TestCases/Pointer/align-assume-ignorelist.cpp

-// RUN: %clang -fsanitize=alignment -fno-sanitize-recover=alignment                           -O0 %s -o %t && not %run %t 2>&1 | FileCheck %s --implicit-check-not=" assumption "
+// RUN: %clangxx -fsanitize=alignment -fno-sanitize-recover=alignment                           -O0 %s -o %t && not %run %t 2>&1 | FileCheck %s --implicit-check-not=" assumption "
 
 // RUN: rm -f %tmp
 // RUN: echo "[alignment]" >> %tmp
 // RUN: echo "fun:main" >> %tmp
-// RUN: %clang -fsanitize=alignment -fno-sanitize-recover=alignment -fsanitize-ignorelist=%tmp -O0 %s -o %t && %run %t 2>&1
+// RUN: %clangxx -fsanitize=alignment -fno-sanitize-recover=alignment -fsanitize-ignorelist=%tmp -O0 %s -o %t && %run %t 2>&1
 
 #include <stdlib.h>
 
 int main(int argc, char* argv[]) {
   char *ptr = (char *)malloc(2);
 
   __builtin_assume_aligned(ptr + 1, 0x8000);
   // CHECK: {{.*}}align-assume-ignorelist.cpp:[[@LINE-1]]:32: runtime error: assumption of 32768 byte alignment for pointer of type 'char *' failed
   // CHECK: 0x{{.*}}: note: address is {{.*}} aligned, misalignment offset is {{.*}} byte
 
   free(ptr);
 
   return 0;
 }