[libc] Fix problem with using isnan in test/src/math/logf_test.cpp.

[lntue]

Summary

The default isnan returns an int, and hence failing the EXPECT_TRUE tests.

libc/test/src/math/logf_test.cpp

 
 namespace mpfr = __llvm_libc::testing::mpfr;
 
 DECLARE_SPECIAL_CONSTANTS(float)
 
 TEST(LlvmLibcLogfTest, SpecialNumbers) {
   EXPECT_FP_EQ(aNaN, __llvm_libc::logf(aNaN));
   EXPECT_FP_EQ(inf, __llvm_libc::logf(inf));
-  EXPECT_TRUE(isnan(__llvm_libc::logf(neg_inf)));
+  EXPECT_TRUE(FPBits((__llvm_libc::logf(neg_inf))).is_nan());
   EXPECT_FP_EQ(neg_inf, __llvm_libc::logf(0.0f));
   EXPECT_FP_EQ(neg_inf, __llvm_libc::logf(-0.0f));
-  EXPECT_TRUE(isnan(__llvm_libc::logf(-1.0f)));
+  EXPECT_TRUE(FPBits(__llvm_libc::logf(-1.0f)).is_nan());
   EXPECT_FP_EQ(zero, __llvm_libc::logf(1.0f));
 }
 
 TEST(LlvmLibcLogfTest, TrickyInputs) {
   constexpr int N = 24;
   constexpr uint32_t INPUTS[N] = {
       0x3509dcf6U, 0x3bf86ef0U, 0x3ca1c99fU, 0x3d13e105U, 0x3f7ff1f2U,
       0x3f7fffffU, 0x3f800006U, 0x3f800014U, 0x3f80001cU, 0x3f80c777U,