[flang] Fix -Wcharacter-conversion warnings (NFC) (#155873)

This fixes new warnings when building with Clang 21, encountered in
#155627.

Author: nikic

flang/lib/Evaluate/fold-implementation.h

@@ -1785,7 +1785,7 @@ common::IfNoLvalue<std::optional<TO>, FROM> ConvertString(FROM &&s) {
       if (static_cast<std::uint64_t>(*iter) > 127) {
         return std::nullopt;
       }
-      str.push_back(*iter);
+      str.push_back(static_cast<typename TO::value_type>(*iter));
     }
     return std::make_optional<TO>(std::move(str));
   }

flang/lib/Parser/characters.cpp

@@ -289,7 +289,8 @@ RESULT DecodeString(const std::string &s, bool backslashEscapes) {
         DecodeCharacter<ENCODING>(p, bytes, backslashEscapes)};
     if (decoded.bytes > 0) {
       if (static_cast<std::size_t>(decoded.bytes) <= bytes) {
-        result.append(1, decoded.codepoint);
+        result.append(
+            1, static_cast<typename RESULT::value_type>(decoded.codepoint));
         bytes -= decoded.bytes;
         p += decoded.bytes;
         continue;