Testscases for llvm/test/FileCheck/simple-var-capture.txt, about variables with backslashes

simple-var-capture.txt

// Variable should be matched to backslashes.
posix: /path/to/posix/file
dos:   c:\path\to\dos\file
dosx:  d:\\path\\to\\escaped\\file
CHECK: posix: [[POSIX:.*]]/file
CHECK: dos: [[DOS:.*]]/file
CHECK: posix: [[DOSX:.*]]/file

POSIX should be matched to /path/to/posix/file(/path/to/posix)
DOS should be matched to c:\path\to\dos\file(c:\path\to\dos)
DOSX should be matched to d:\\path\\to\\escaped\\file(d:\\path\\to\\escaped)
CHECK: [[POSIX]]/file([[POSIX]])
CHECK: [[DOS]]\file([[DOS]])
CHECK: [[DOSX]]\\file([[DOSX]])