Allow backslashes as path separators; should fix tests on Windows

Author: kovdan01

llvm/test/FileCheck/empty-variable-name.txt

@@ -5,7 +5,7 @@ a
 ; RUN:   --match-full-lines --strict-whitespace %s
 
 ; CHECK:        a[[]]
-; CHECK-ERROR:[[DIR]]/empty-variable-name.txt:7:13: error: empty variable name
+; CHECK-ERROR:[[DIR]]{{/|\\}}empty-variable-name.txt:7:13: error: empty variable name
 ; CHECK-ERROR-NEXT:; CHECK: a{{\[\[\]\]}}
 ;      CHECK-ERROR-NEXT:            ^
 
@@ -16,7 +16,7 @@ b
 ; RUN:   --match-full-lines --strict-whitespace %s
 
 ; CHECK-PSEUDO: b[[@]]
-; CHECK-ERROR-PSEUDO:[[DIR]]/empty-variable-name.txt:18:21: error: empty pseudo variable name
+; CHECK-ERROR-PSEUDO:[[DIR]]{{/|\\}}empty-variable-name.txt:18:21: error: empty pseudo variable name
 ; CHECK-ERROR-PSEUDO-NEXT:; CHECK-PSEUDO: b{{\[\[@\]\]}}
 ;      CHECK-ERROR-PSEUDO-NEXT:                    ^
 
@@ -27,6 +27,6 @@ c
 ; RUN:   --match-full-lines --strict-whitespace %s
 
 ; CHECK-GLOBAL: c[[$]]
-; CHECK-ERROR-GLOBAL:[[DIR]]/empty-variable-name.txt:29:21: error: empty global variable name
+; CHECK-ERROR-GLOBAL:[[DIR]]{{/|\\}}empty-variable-name.txt:29:21: error: empty global variable name
 ; CHECK-ERROR-GLOBAL-NEXT:; CHECK-GLOBAL: c{{\[\[\$\]\]}}
 ;       CHECK-ERROR-GLOBAL-NEXT:                    ^