Switch SourceBuffer to diagnostics. (#3197)

This updates SourceBuffer to diagnostics. Some additional edits to
diagnostics were necessary due to issues moving arguments around, which
seems to stem from a compile error with clang 14 (fixed in later
versions).

---------

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

toolchain/lex/tokenized_buffer_test.cpp

@@ -34,8 +34,8 @@ class LexerTest : public ::testing::Test {
     std::string filename = llvm::formatv("test{0}.carbon", ++file_index_);
     CARBON_CHECK(fs_.addFile(filename, /*ModificationTime=*/0,
                              llvm::MemoryBuffer::getMemBuffer(text)));
-    source_storage_.push_front(
-        std::move(*SourceBuffer::CreateFromFile(fs_, llvm::errs(), filename)));
+    source_storage_.push_front(std::move(*SourceBuffer::CreateFromFile(
+        fs_, filename, ConsoleDiagnosticConsumer())));
     return source_storage_.front();
   }