Correct a comment and update a return type; NFC

These changes were inspired by a post-commit review comment:
#97274 (review)

clang/include/clang/Lex/Preprocessor.h

@@ -2139,8 +2139,8 @@ class Preprocessor {
   }
 
   /// Given a Token \p Tok that is a numeric constant with length 1,
-  /// return the character.
-  char
+  /// return the value of constant as an unsigned 8-bit integer.
+  uint8_t
   getSpellingOfSingleCharacterNumericConstant(const Token &Tok,
                                               bool *Invalid = nullptr) const {
     assert((Tok.is(tok::numeric_constant) || Tok.is(tok::binary_data)) &&

clang/lib/Sema/SemaExpr.cpp

@@ -3652,7 +3652,7 @@ ExprResult Sema::ActOnNumericConstant(const Token &Tok, Scope *UDLScope) {
   // Fast path for a single digit (which is quite common).  A single digit
   // cannot have a trigraph, escaped newline, radix prefix, or suffix.
   if (Tok.getLength() == 1 || Tok.getKind() == tok::binary_data) {
-    const char Val = PP.getSpellingOfSingleCharacterNumericConstant(Tok);
+    const uint8_t Val = PP.getSpellingOfSingleCharacterNumericConstant(Tok);
     return ActOnIntegerConstant(Tok.getLocation(), Val);
   }