[libc][NFC] Fix str_to_float and uint warnings

[michaelrj]

Summary

More Wconversion and Wno-sign-conversion warning fixes.

libc/src/__support/UInt.h

if (other == 1) {
       return remainder;
     }
     if (other == 0) {
       return nullopt;
     }
 
     BigInt<Bits, Signed> quotient(0);
     BigInt<Bits, Signed> subtractor = other;
-    int cur_bit = subtractor.clz() - this->clz();
+    int cur_bit = static_cast<int>(subtractor.clz() - this->clz());
     subtractor.shift_left(cur_bit);
 
     for (; cur_bit >= 0 && *this > 0; --cur_bit, subtractor.shift_right(1)) {
       if (*this >= subtractor) {
         this->sub(subtractor);
         quotient = quotient | (BigInt<Bits, Signed>(1) << cur_bit);
       }
     }

libc/src/__support/str_to_float.h

if (tolower(src[index + 1]) == nan_string[1] &&
             // input type "NAN(n-char-sequence)" that "the meaning of
             // the n-char sequence is implementation-defined."
 
             auto strtoint_result =
                 strtointeger<uint64_t>(src + (left_paren + 1), 0);
             if (strtoint_result.has_error()) {
               error = strtoint_result.error;
             }
-            nan_mantissa = strtoint_result.value;
+            nan_mantissa = static_cast<BitsType>(strtoint_result.value);
             if (src[left_paren + 1 + strtoint_result.parsed_len] != ')')
               nan_mantissa = 0;
           }
         } else {
           index = left_paren;
         }
       }
       nan_mantissa |= fputil::FloatProperties<T>::QUIET_NAN_MASK;