Disable bugprone-unchecked-optional-access due to false positives (#3553)

I think this check doesn't offer enough value to try to work around the
false positives. We probably will, at times, check the contents of an
optional without validating because structurally we know it must have a
value. Here though, I believe the `while` will be doing a check of the
value; we shouldn't need a more explicit check.

.clang-tidy

@@ -5,6 +5,12 @@
 ---
 # - bugprone-exception-escape finds issues like out-of-memory in main(). We
 #   don't use exceptions, so it's unlikely to find real issues.
+# - bugprone-unchecked-optional-access in clang-tidy 16 has false positives on
+#   code like:
+#     while (auto name_ref = insts().Get(inst_id).TryAs<SemIR::NameRef>()) {
+#       inst_id = name_ref->value_id;
+#                 ^ unchecked access to optional value
+#     }
 # - google-readability-function-size overlaps with readability-function-size.
 # - modernize-use-nodiscard is disabled because it only fixes const methods,
 #   not non-const, which yields distracting results on accessors.
@@ -12,7 +18,8 @@
 #   modernize-pass-by-value.
 Checks:
   -*, bugprone-*, -bugprone-branch-clone, -bugprone-easily-swappable-parameters,
-  -bugprone-exception-escape, -bugprone-narrowing-conversions, google-*,
+  -bugprone-exception-escape, -bugprone-narrowing-conversions,
+  -bugprone-unchecked-optional-access, google-*,
   -google-readability-function-size, -google-readability-todo,
   misc-definitions-in-headers, misc-misplaced-const, misc-redundant-expression,
   misc-static-assert, misc-unconventional-assign-operator,