Fix external-names.c test when separator is \\

This fixes the following failure:

C:\[...]\llvm\tools\clang\test\VFS\external-names.c:34:26: error: CHECK-DEBUG-EXTERNAL: expected string not found in input
// CHECK-DEBUG-EXTERNAL: ![[Num]] = !DIFile(filename: "{{[^"]*}}Inputs{{.}}external-names.h"
                         ^
[...]
<stdin>:42:54: note: possible intended match here
!10 = !DIFile(filename: "C:/[...]\\llvm\\tools\\clang\\test\\VFS\\Inputs\\external-names.h", directory: "")

Differential Revision: https://reviews.llvm.org/D71991