[ubsan] Stop cast-overflow.cpp test leaking undefined behaviour into the exit code.

[dsanders]

Summary

ubsan was correctly catching the undefined behaviour but lit's shell was
failing the test anyway because the exit code was non-zero as a result of the
undefined behaviour.

This fixes the test on a mips-linux-gnu target.

[samsonov]

LGTM

test/ubsan/TestCases/Float/cast-overflow.cpp

switch (argv[1][0]) {
     // Floating point -> integer overflow.
   case '0': {
     // Note that values between 0x7ffffe00 and 0x80000000 may or may not
     // successfully round-trip, depending on the rounding mode.
     // CHECK-0: runtime error: value 2.14748{{.*}} is outside the range of representable values of type 'int'
     static int test_int = MaxFloatRepresentableAsInt + 0x80;
     return 0;
     }
-  case '1':
+  case '1': {
     // CHECK-1: runtime error: value -2.14748{{.*}} is outside the range of representable values of type 'int'
-    return MinFloatRepresentableAsInt - 0x100;
+    static int test_int = MinFloatRepresentableAsInt - 0x100;
+    return 0;
+  }
   case '2': {
     // CHECK-2: runtime error: value -1 is outside the range of representable values of type 'unsigned int'
     volatile float f = -1.0;
     volatile unsigned u = (unsigned)f;
     return 0;
   }
   case '3': {
     // CHECK-3: runtime error: value 4.2949{{.*}} is outside the range of representable values of type 'unsigned int'
     static int test_int = (unsigned)(MaxFloatRepresentableAsUInt + 0x100);
     return 0;
   }
 
   case '4': {
     // CHECK-4: runtime error: value {{.*}} is outside the range of representable values of type 'int'
     static int test_int = Inf;
     return 0;
   }
-  case '5':
+  case '5': {
     // CHECK-5: runtime error: value {{.*}} is outside the range of representable values of type 'int'
-    return NaN;
+    static int test_int = NaN;
+    return 0;
+  }
 
     // Integer -> floating point overflow.
   case '6': {
     // CHECK-6: {{runtime error: value 0xffffff00000000000000000000000001 is outside the range of representable values of type 'float'|__int128 not supported}}
 #ifdef __SIZEOF_INT128__
     static int test_int = (float)(FloatMaxAsUInt128 + 1);
     return 0;
 #else