test: adjust declaration to be portable (NFCI)

`long` is teated as `Int32` on Windows x86_64.  `intptr_t` will be
mapped to `Int` and `Int64`.  This fixes the failing test on Windows
which is LLP64 and should be equivalent on LP64 targets.

Author: compnerd

test/ClangImporter/Inputs/custom-modules/UnimportableMembersUser.h

@@ -1,5 +1,5 @@
 @import UnimportableMembers;
 
 @interface DesignatedInitializerInAnotherModule (/*evil class extension*/)
-- (instancetype)initFromOtherModule:(long)x __attribute__((objc_designated_initializer));
+- (instancetype)initFromOtherModule:(intptr_t)x __attribute__((objc_designated_initializer));
 @end