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_fuzzer.cpp

@@ -83,7 +83,10 @@ extern "C" auto LLVMFuzzerTestOneInput(const unsigned char* data, size_t size)
   llvm::raw_null_ostream dest;
   Driver d(fs, install_paths, dest, error_stream);
   if (!d.RunCommand(args).success) {
-    if (error_stream.TakeStr().find("ERROR:") == std::string::npos) {
+    auto str = error_stream.TakeStr();
+    // TODO: Fix command_line to use `error`, switch back to `find`.
+    if (llvm::StringRef(str).find_insensitive("error:") ==
+        llvm::StringRef::npos) {
       llvm::errs() << "No error message on a failure!\n";
       return 1;
     }