Style updates, mostly _ naming (#970)

There are some declaration order changes, and a few test classes switched from `struct` to `class`. However, this PR is mostly adopting `_` naming of private member variables due to the shift in naming style. None of what's here should have behavior impacts, it should just be style.

Note, there are a lot of things that *look* like they could be accessor-named, but I'm not doing that in this change. Happy to do it separately if you want me to do another PR focused on it.

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

toolchain/lexer/tokenized_buffer_test.cpp

@@ -34,9 +34,8 @@ using ::testing::HasSubstr;
 using ::testing::StrEq;
 namespace Yaml = Carbon::Testing::Yaml;
 
-struct LexerTest : ::testing::Test {
-  llvm::SmallVector<SourceBuffer, 16> source_storage;
-
+class LexerTest : public ::testing::Test {
+ protected:
   auto GetSourceBuffer(llvm::Twine text) -> SourceBuffer& {
     source_storage.push_back(SourceBuffer::CreateFromText(text.str()));
     return source_storage.back();
@@ -47,6 +46,8 @@ struct LexerTest : ::testing::Test {
       -> TokenizedBuffer {
     return TokenizedBuffer::Lex(GetSourceBuffer(text), consumer);
   }
+
+  llvm::SmallVector<SourceBuffer, 16> source_storage;
 };
 
 TEST_F(LexerTest, HandlesEmptyBuffer) {