Update basic diagnostic capitalization/punctuation (#4328)

This is a primarily automated change:

- Search & replace for capitalization
-
`(CARBON_DIAGNOSTIC\((?:\n\s+)?\w+,(?:\n\s+)?\s\w+,(?:\n\s+)?\s")([A-Z])`
    - `$1\L$2`
- Search & replace for period
-
`(CARBON_DIAGNOSTIC\((?:\n\s+)?\w+,(?:\n\s+)?\s\w+,(?:\n\s+)?\s"(?:[^)]|\n)+)\.("[,)])`
    - `$1$2`
- Limited search & replace for `ERROR: ` -> `error: ` in streamed things
- Leaving a TODO for command_line because there's more cleanup that can
be done there
- Modify diagnostic_consumer.cpp
    - ERROR -> error
    - WARNING -> warning

---------

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

toolchain/parse/handle_function.cpp

@@ -57,7 +57,7 @@ auto HandleFunctionSignatureFinish(Context& context) -> void {
       if (!context.ConsumeAndAddLeafNodeIf(Lex::TokenKind::StringLiteral,
                                            NodeKind::BuiltinName)) {
         CARBON_DIAGNOSTIC(ExpectedBuiltinName, Error,
-                          "Expected builtin function name after `=`.");
+                          "expected builtin function name after `=`");
         context.emitter().Emit(*context.position(), ExpectedBuiltinName);
         state.has_error = true;
       }