Support lexing characters (#5893)

Adapts `StringLiteral` to lex characters. Adds a `CharLiteral` token,
which contains a `CharLiteralValue` which is a straight unicode code
point (suggested by zygoloid).

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/lex/string_literal_test.cpp

@@ -32,7 +32,7 @@ class StringLiteralTest : public ::testing::Test {
   auto Parse(llvm::StringRef text) -> llvm::StringRef {
     StringLiteral token = Lex(text);
     Testing::SingleTokenDiagnosticEmitter emitter(&error_tracker_, text);
-    return token.ComputeValue(allocator_, emitter);
+    return token.ComputeStringValue(allocator_, emitter);
   }
 
   llvm::BumpPtrAllocator allocator_;