Run clang-tidy on headers (#996)

This is a largely mechanical PR that extends our `clang-tidy` setup to
run on header files as well as implementations (an oversight in our
earlier introduction of the checker).

No new checks are added, so there shouldn't be anything controversial in
the changes here.

Fixes #995

.clang-tidy

@@ -21,6 +21,7 @@ Checks:
   -bugprone-exception-escape
   -bugprone-implicit-widening-of-multiplication-result
   -bugprone-unchecked-optional-access
+  -bugprone-lambda-function-name
   cppcoreguidelines-*
   -cppcoreguidelines-avoid-magic-numbers
   -cppcoreguidelines-avoid-non-const-global-variables
@@ -43,3 +44,5 @@ CheckOptions:
     value: true
   - key: 'cppcoreguidelines-macro-usage.CheckCapsOnly'
     value: true
+  - key: 'cppcoreguidelines-special-member-functions.AllowSoleDefaultDtor'
+    value: true