clang-tidy: add more bugprone nits

As discussed on
https://groups.google.com/a/chromium.org/g/cxx/c/iow8Ugo7190, adding
these sounds like a good idea.

Bug: None
Change-Id: Ic7a721d8c94e5e5f0629bdc50b6d466c7b344ab6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438310
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812863}

.clang-tidy

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   Checks:          '-*,
+                    bugprone-string-integer-assignment,
+                    bugprone-unused-raii,
                     bugprone-use-after-move,
                     google-build-explicit-make-pair,
                     google-explicit-constructor,