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/driver/driver_test.cpp

@@ -22,21 +22,11 @@ namespace Yaml = Carbon::Testing::Yaml;
 
 /// A raw_ostream that makes it easy to repeatedly check streamed output.
 class RawTestOstream : public llvm::raw_ostream {
-  std::string buffer;
-
-  void write_impl(const char* ptr, size_t size) override {
-    buffer.append(ptr, ptr + size);
-  }
-
-  [[nodiscard]] auto current_pos() const -> uint64_t override {
-    return buffer.size();
-  }
-
  public:
   ~RawTestOstream() override {
     flush();
-    if (!buffer.empty()) {
-      ADD_FAILURE() << "Unchecked output:\n" << buffer;
+    if (!buffer_.empty()) {
+      ADD_FAILURE() << "Unchecked output:\n" << buffer_;
     }
   }
 
@@ -44,10 +34,21 @@ class RawTestOstream : public llvm::raw_ostream {
   /// back to empty.
   auto TakeStr() -> std::string {
     flush();
-    std::string result = std::move(buffer);
-    buffer.clear();
+    std::string result = std::move(buffer_);
+    buffer_.clear();
     return result;
   }
+
+ private:
+  void write_impl(const char* ptr, size_t size) override {
+    buffer_.append(ptr, ptr + size);
+  }
+
+  [[nodiscard]] auto current_pos() const -> uint64_t override {
+    return buffer_.size();
+  }
+
+  std::string buffer_;
 };
 
 TEST(DriverTest, FullCommandErrors) {