Support parse/check/lower for char (#5901)

toolchain/check/testdata/builtins/char/basics.carbon and
toolchain/lower/testdata/builtins/char.carbon are probably the most
interesting tests here. The parse tests is required because this adds a
new node kind, and we need coverage of it; but the attached info is
minor. There's a fair amount of test churn here because I'm adding the
Core.Char and Core.CharLiteral types as new singletons.

My intent here is that `CharId` is always a unicode code point, even
when the type is a `Char` and thus must be a single UTF-8 code unit
(single byte). This mainly means the stored value of a `CharValue` can
be printed internally without knowing the type.

---------

Co-authored-by: josh11b <15258583+josh11b@users.noreply.github.com>
Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/lex/string_literal.cpp

@@ -476,8 +476,9 @@ static auto ExpandEscapeSequencesAndRemoveIndent(
   }
 }
 
-auto StringLiteral::ComputeCharValue(Diagnostics::Emitter<const char*>& emitter)
-    const -> std::optional<CharLiteralValue> {
+auto StringLiteral::ComputeCharLiteralValue(
+    Diagnostics::Emitter<const char*>& emitter) const
+    -> std::optional<CharLiteralValue> {
   CARBON_DCHECK(kind_ == Kind::Char);
   CARBON_DCHECK(is_terminated_);