Revert changes to llvm-string stdin test and XFAIL on AIX (#160821)

Xfail testcase on AIX until echo -n bug is fixed, revert changes made to
testcase

Author: madanial0

llvm/test/tools/llvm-strings/stdin.test

@@ -1,13 +1,13 @@
+# XFAIL: system-aix
+
 ## Show that llvm-strings can handle stdin input properly.
 
 ## Case 1: output with single string.
-RUN: echo -n "abcdefg" > %t
-RUN: llvm-strings %t - | FileCheck %s --check-prefix=CASE1 --implicit-check-not={{.}}
+RUN: echo -n "abcdefg" | llvm-strings - | FileCheck %s --check-prefix=CASE1 --implicit-check-not={{.}}
 CASE1: abcdefg
 
 ## Case 2: output too short for string.
-RUN: echo -n "abc" > %t
-RUN: llvm-strings %t - | FileCheck %s --implicit-check-not={{.}} --allow-empty
+RUN: echo -n "abc" | llvm-strings - | FileCheck %s --implicit-check-not={{.}} --allow-empty
 
 ## Case 3: output with new line.
 RUN: printf "abcd\nefgh" | llvm-strings - | FileCheck %s --check-prefix=CASE3 --implicit-check-not={{.}}
@@ -21,5 +21,4 @@ CASE4-NEXT: ghij
 CASE4-NEXT: klmn
 
 ## Case 5: no file name specified is equivalent to explicitly requesting stdin.
-RUN: echo -n "abcdefg" > %t
-RUN: llvm-strings %t - | FileCheck %s --check-prefix=CASE1 --implicit-check-not={{.}}
+RUN: echo -n "abcdefg" | llvm-strings | FileCheck %s --check-prefix=CASE1 --implicit-check-not={{.}}