Disable modernize-use-designated-initializers check for now. (#3770)

This check is adding a lot of noise to the clang-tidy output for
construction of `SemIR::Inst` types, and we don't want to switch to
using designated initialization for them at this time.

.clang-tidy

@@ -19,6 +19,9 @@ WarningsAsErrors: '*'
 # - bugprone-switch-missing-default-case has false positives for `enum_base.h`.
 #   Clang's built-in switch warnings cover most of our risk of bugs here.
 # - google-readability-function-size overlaps with readability-function-size.
+# - modernize-use-designated-initializers is disabled because it fires on
+#   creation of SemIR typed insts, for which we do not currently want to use
+#   designated initialization.
 # - modernize-use-nodiscard is disabled because it only fixes const methods,
 #   not non-const, which yields distracting results on accessors.
 # - performance-unnecessary-value-param is disabled because it duplicate
@@ -32,9 +35,10 @@ Checks:
   misc-static-assert, misc-unconventional-assign-operator,
   misc-uniqueptr-reset-release, misc-unused-*, modernize-*,
   -modernize-avoid-c-arrays, -modernize-return-braced-init-list,
-  -modernize-use-default-member-init, -modernize-use-emplace,
-  -modernize-use-nodiscard, performance-*, -performance-unnecessary-value-param,
-  readability-*, -readability-convert-member-functions-to-static,
+  -modernize-use-default-member-init, -modernize-use-designated-initializers,
+  -modernize-use-emplace, -modernize-use-nodiscard, performance-*,
+  -performance-unnecessary-value-param, readability-*,
+  -readability-convert-member-functions-to-static,
   -readability-function-cognitive-complexity, -readability-else-after-return,
   -readability-identifier-length, -readability-implicit-bool-conversion,
   -readability-magic-numbers, -readability-make-member-function-const,