[libc] build fix: always use our char8_t headers even in overlay mode (#144433)

Build fix caused by certain platforms not providing char8_t when
expected
Temporary fix to just always use our own definition, even in overlay
mode.

Author: uzairnawaz

libc/hdr/types/char8_t.h

@@ -9,14 +9,6 @@
 #ifndef LLVM_LIBC_HDR_TYPES_CHAR8_T_H
 #define LLVM_LIBC_HDR_TYPES_CHAR8_T_H
 
-#ifdef LIBC_FULL_BUILD
-
 #include "include/llvm-libc-types/char8_t.h"
 
-#else // overlay mode
-
-#include "hdr/uchar_overlay.h"
-
-#endif // LLVM_LIBC_FULL_BUILD
-
 #endif // LLVM_LIBC_HDR_TYPES_CHAR8_T_H

libc/src/__support/wchar/character_converter.cpp

@@ -70,7 +70,7 @@ ErrorOr<char8_t> CharacterConverter::pop_utf8() {
   char32_t output;
 
   // Shift to get the next 6 bits from the utf32 encoding
-  const char32_t shift_amount =
+  const size_t shift_amount =
       (state->total_bytes - state->bytes_processed - 1) * ENCODED_BITS_PER_UTF8;
   if (state->bytes_processed == 0) {
     /*