[libc] Fix -Wimplicit-int-conversion warnings

Differential Revision: https://reviews.llvm.org/D140492

Author: abrachet

libc/src/__support/StringUtil/message_mapper.h

@@ -45,7 +45,7 @@ template <size_t ARR_SIZE, size_t TOTAL_STR_LEN> class MessageMapper {
     for (size_t i = 0; i < raw_array_len; ++i)
       string_mappings[raw_array[i].num] = raw_array[i].msg;
 
-    size_t string_array_index = 0;
+    int string_array_index = 0;
     for (size_t cur_num = 0; cur_num < ARR_SIZE; ++cur_num) {
       if (string_mappings[cur_num].size() != 0) {
         msg_offsets[cur_num] = string_array_index;

libc/src/__support/integer_to_string.h

@@ -59,7 +59,7 @@ class IntegerToString {
     } else {
       for (; uval > 0; --buffptr, uval /= conv_base) {
         uintmax_t digit = (uval % conv_base);
-        buffer[buffptr - 1] = digit < 10 ? digit + '0' : digit + a - 10;
+        buffer[buffptr - 1] = static_cast<char>(digit < 10 ? digit + '0' : digit + a - 10);
       }
     }
     len = buffer.size() - buffptr;