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/string_literal_test.cpp

@@ -14,11 +14,10 @@
 namespace Carbon {
 namespace {
 
-struct StringLiteralTest : ::testing::Test {
+class StringLiteralTest : public ::testing::Test {
+ protected:
   StringLiteralTest() : error_tracker(ConsoleDiagnosticConsumer()) {}
 
-  ErrorTrackingDiagnosticConsumer error_tracker;
-
   auto Lex(llvm::StringRef text) -> LexedStringLiteral {
     llvm::Optional<LexedStringLiteral> result = LexedStringLiteral::Lex(text);
     assert(result);
@@ -32,6 +31,8 @@ struct StringLiteralTest : ::testing::Test {
     DiagnosticEmitter<const char*> emitter(translator, error_tracker);
     return token.ComputeValue(emitter);
   }
+
+  ErrorTrackingDiagnosticConsumer error_tracker;
 };
 
 TEST_F(StringLiteralTest, StringLiteralBounds) {