[libc][nfc] Silence two warnings in tests

[jhuber6]

Summary

These currently give warnings for unused variables or a default case
where everything is covered.

libc/test/UnitTest/RoundingModeUtils.cpp

 namespace __llvm_libc {
 namespace fputil {
 namespace testing {
 
 int get_fe_rounding(RoundingMode mode) {
   switch (mode) {
   case RoundingMode::Upward:
     return FE_UPWARD;
-    break;
   case RoundingMode::Downward:
     return FE_DOWNWARD;
-    break;
   case RoundingMode::TowardZero:
     return FE_TOWARDZERO;
-    break;
   case RoundingMode::Nearest:
     return FE_TONEAREST;
-    break;
-  default:
-    __builtin_unreachable();
   }
+  __builtin_unreachable();
 }
 
 ForceRoundingMode::ForceRoundingMode(RoundingMode mode) {
   old_rounding_mode = quick_get_round();
   rounding_mode = get_fe_rounding(mode);
   if (old_rounding_mode != rounding_mode) {
     int status = set_round(rounding_mode);
     success = (status == 0);

libc/test/src/__support/uint_test.cpp

 using LL_UInt320 = __llvm_libc::cpp::UInt<320>;
 using LL_UInt512 = __llvm_libc::cpp::UInt<512>;
 using LL_UInt1024 = __llvm_libc::cpp::UInt<1024>;
 
 using LL_Int128 = __llvm_libc::cpp::Int<128>;
 using LL_Int192 = __llvm_libc::cpp::Int<192>;
 
 TEST(LlvmLibcUIntClassTest, BasicInit) {
-  LL_UInt128 empty;
   LL_UInt128 half_val(12345);
   LL_UInt128 full_val({12345, 67890});
   ASSERT_TRUE(half_val != full_val);
 }
 
 TEST(LlvmLibcUIntClassTest, AdditionTests) {
   LL_UInt128 val1(12345);
   LL_UInt128 val2(54321);