Revert "PrintAsClang: Ignore nullability attributes for C clients"

This reverts commit 20e1ac6.

Author: hamishknight

lib/PrintAsClang/PrintAsClang.cpp

@@ -218,21 +218,6 @@ static void writePrologue(raw_ostream &out, ASTContext &ctx,
 
   static_assert(SWIFT_MAX_IMPORTED_SIMD_ELEMENTS == 4,
               "need to add SIMD typedefs here if max elements is increased");
-
-  if (ctx.LangOpts.hasFeature(Feature::CDecl)) {
-    // For C compilers which don’t support nullability attributes, ignore them;
-    // for ones which do, suppress warnings about them being an extension.
-    out << "#if !__has_feature(nullability)\n"
-           "# define _Nonnull\n"
-           "# define _Nullable\n"
-           "# define _Null_unspecified\n"
-           "#elif !defined(__OBJC__)\n"
-           "# pragma clang diagnostic ignored \"-Wnullability-extension\"\n"
-           "#endif\n"
-           "#if !__has_feature(nullability_nullable_result)\n"
-           "# define _Nullable_result _Nullable\n"
-           "#endif\n";
-  }
 }
 
 static int compareImportModulesByName(const ImportModuleTy *left,