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/driver/driver_test.cpp

@@ -129,7 +129,7 @@ TEST_F(DriverTest, CompileCommandErrors) {
                                 "--output=/dev/empty", empty_file})
                    .success);
   EXPECT_THAT(test_error_stream_.TakeStr(),
-              ContainsRegex("ERROR: .*/dev/empty.*"));
+              ContainsRegex("error: .*/dev/empty.*"));
 }
 
 TEST_F(DriverTest, DumpTokens) {