Enable misc-non-private-member-variables-in-classes and adjust style to match (#4702)

Pursuant to discussion regarding #4699, turn on
`misc-non-private-member-variables-in-classes` using the
`IgnoreClassesWithAllMemberVariablesBeingPublic` flag (the check treats
structs as classes, so we need this for structs with all-public
members). Updates the style guide notes to match, which should be pretty
minor due to the scoping of test fixtures.

Also fixes some underscore uses in test files on the way. Basically this
is keeping the style for [class data member
naming](https://google.github.io/styleguide/cppguide.html#Variable_Names)
even while making them public.

toolchain/lex/tokenized_buffer_test.cpp

@@ -37,7 +37,7 @@ using ::testing::Pair;
 namespace Yaml = ::Carbon::Testing::Yaml;
 
 class LexerTest : public ::testing::Test {
- protected:
+ public:
   Testing::CompileHelper compile_helper_;
 };