[flang-rt] Fix warnings

This patch fixes:

  flang-rt/include/flang-rt/runtime/emit-encoded.h:67:27: error:
  implicit conversion from 'const char16_t' to 'char32_t' may change
  the meaning of the represented code unit
  [-Werror,-Wcharacter-conversion]

  flang-rt/lib/runtime/edit-input.cpp:1114:18: error: implicit
  conversion from 'char32_t' to 'char16_t' may lose precision and
  change the meaning of the represented code unit
  [-Werror,-Wcharacter-conversion]

  flang-rt/lib/runtime/edit-input.cpp:1133:18: error: implicit
  conversion from 'char32_t' to 'char16_t' may lose precision and
  change the meaning of the represented code unit
  [-Werror,-Wcharacter-conversion]

  flang-rt/lib/runtime/edit-input.cpp:1033:14: error: implicit
  conversion from 'char32_t' to 'char16_t' may lose precision and
  change the meaning of the represented code unit
  [-Werror,-Wcharacter-conversion]

  flang-rt/lib/runtime/edit-input.cpp:986:14: error: implicit
  conversion from 'char32_t' to 'char16_t' may lose precision and
  change the meaning of the represented code unit
  [-Werror,-Wcharacter-conversion]

Author: kazutakahirata

flang-rt/include/flang-rt/runtime/emit-encoded.h

@@ -64,7 +64,7 @@ RT_API_ATTRS bool EmitEncoded(
     } else {
       // CHARACTER kind conversion for internal output
       while (chars-- > 0) {
-        char32_t buffer = *data++;
+        char32_t buffer = static_cast<char32_t>(*data++);
         char *p{reinterpret_cast<char *>(&buffer)};
         if constexpr (!isHostLittleEndian) {
           p += sizeof(buffer) - internalKind;

flang-rt/lib/runtime/edit-input.cpp

@@ -983,7 +983,7 @@ static RT_API_ATTRS bool EditDelimitedCharacterInput(
       }
     }
     if (length > 0) {
-      *x++ = *ch;
+      *x++ = static_cast<CHAR>(*ch);
       --length;
     }
   }
@@ -1030,7 +1030,7 @@ static RT_API_ATTRS bool EditListDirectedCharacterInput(
       break;
     }
     if (length > 0) {
-      *x++ = *ch;
+      *x++ = static_cast<CHAR>(*ch);
       --length;
     } else if (edit.IsNamelist()) {
       // GNU compatibility
@@ -1111,7 +1111,7 @@ RT_API_ATTRS bool EditCharacterInput(IoStatementState &io, const DataEdit &edit,
             (sizeof *x == 2 && *ucs > 0xffff)) {
           *x++ = '?';
         } else {
-          *x++ = *ucs;
+          *x++ = static_cast<CHAR>(*ucs);
         }
         --lengthChars;
       } else if (chunkBytes == 0) {
@@ -1130,7 +1130,7 @@ RT_API_ATTRS bool EditCharacterInput(IoStatementState &io, const DataEdit &edit,
             (sizeof *x == 2 && buffer > 0xffff)) {
           *x++ = '?';
         } else {
-          *x++ = buffer;
+          *x++ = static_cast<CHAR>(buffer);
         }
         --lengthChars;
       }