[libc][NFC] moving template specialization outside class declaration

[gchatelet]

Summary

This is necessary to get llvm-libc compile with GCC.
This patch is extracted from D119002.

libc/utils/MPFRWrapper/MPFRUtils.cpp

std::string str() const {
     cpp::StringView view(buffer);
     view = view.trim(' ');
     return std::string(view.data());
   }
 
   // These functions are useful for debugging.
   template <typename T> T as() const;
 
-  template <> float as<float>() const {
-    return mpfr_get_flt(value, mpfr_rounding);
-  }
-  template <> double as<double>() const {
-    return mpfr_get_d(value, mpfr_rounding);
-  }
-  template <> long double as<long double>() const {
-    return mpfr_get_ld(value, mpfr_rounding);
-  }
-
   void dump(const char *msg) const { mpfr_printf("%s%.128Rf\n", msg, value); }
 
   // Return the ULP (units-in-the-last-place) difference between the
   // stored MPFR and a floating point number.
   //
   // We define ULP difference as follows:
   //   If exponents of this value and the |input| are same, then:
   //     ULP(this_value, input) = abs(this_value - input) / eps(input)
mpfr_mul_2si(maxMPFR.value, maxMPFR.value,
                  -maxExponent + int(fputil::MantissaWidth<T>::VALUE),
                  MPFR_RNDN);
 
     mpfr_add(minMPFR.value, minMPFR.value, maxMPFR.value, MPFR_RNDN);
     return minMPFR.as<double>();
   }
 };
 
+template <> float MPFRNumber::as<float>() const {
+  return mpfr_get_flt(value, mpfr_rounding);
+}
+
+template <> double MPFRNumber::as<double>() const {
+  return mpfr_get_d(value, mpfr_rounding);
+}
+
+template <> long double MPFRNumber::as<long double>() const {
+  return mpfr_get_ld(value, mpfr_rounding);
+}
+
 namespace internal {
 
 template <typename InputType>
 cpp::EnableIfType<cpp::IsFloatingPointType<InputType>::Value, MPFRNumber>
 unary_operation(Operation op, InputType input, unsigned int precision,
                 RoundingMode rounding) {
   MPFRNumber mpfrInput(input, precision, rounding);
   switch (op) {