Disable modernize-use-emplace in clang-tidy (#625)

Because we prefer .push_back({...}) over .emplace_back(...), as noted at https://github.com/carbon-language/carbon-lang/pull/615/files#r662540681

.clang-tidy

@@ -9,7 +9,7 @@ Checks:
   misc-redundant-expression, misc-static-assert,
   misc-unconventional-assign-operator, misc-uniqueptr-reset-release,
   misc-unused-*, modernize-*, -modernize-avoid-c-arrays,
-  -modernize-use-default-member-init, performance-*,
+  -modernize-use-default-member-init, -modernize-use-emplace, performance-*,
   readability-braces-around-statements, readability-identifier-naming
 WarningsAsErrors: true
 CheckOptions: