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/parse/parse_fuzzer.cpp

@@ -27,7 +27,8 @@ extern "C" int LLVMFuzzerTestOneInput(const unsigned char* data,
       TestFileName, /*ModificationTime=*/0,
       llvm::MemoryBuffer::getMemBuffer(data_ref, /*BufferName=*/TestFileName,
                                        /*RequiresNullTerminator=*/false)));
-  auto source = SourceBuffer::CreateFromFile(fs, llvm::nulls(), TestFileName);
+  auto source =
+      SourceBuffer::CreateFromFile(fs, TestFileName, NullDiagnosticConsumer());
 
   // Lex the input.
   auto tokens = Lex::TokenizedBuffer::Lex(*source, NullDiagnosticConsumer());