[NFC] XFAIL tests which need visibility in XCOFF objectwriting

on AIX since this is currently unsupported and we added an error in D124392.

llvm/test/DebugInfo/Generic/pass-by-value.ll

@@ -2,6 +2,10 @@
 ; RUN: %llc_dwarf -O0 -filetype=obj < %s -strict-dwarf=true | llvm-dwarfdump -debug-info - \
 ; RUN:   | FileCheck %s --check-prefix=STRICT --implicit-check-not DW_AT_calling_convention
 ;
+
+; Visibility is unsupported for XCOFF object writing
+; XFAIL: -aix
+
 ; // S is not trivially copyable.
 ; struct S {
 ;    ~S() {}

llvm/test/DebugInfo/cross-cu-scope.ll

@@ -2,6 +2,9 @@
 ; RUN: llvm-dwarfdump -debug-info %t | FileCheck %s
 ; REQUIRES: object-emission
 
+; Visibility is unsupported for XCOFF object writing
+; XFAIL: -aix
+
 ; Reduced test case from PR35212. Two DISubprogram belong to a different CU but
 ; share a scope. Both are declarations and end up in the scope's CU. We want to
 ; check that the CU from the context DIE is used (rather than from the IR).