[clang] [clang][Lex] add -Wnonportable-include-path-separator (PR #186770)

[Aaron Ballman via cfe-commits]

================
@@ -0,0 +1,19 @@
+// REQUIRES: system-windows
+// RUN: mkdir -p %t/backslash
+// RUN: cp %S/Inputs/case-insensitive-include.h %t/backslash/case-insensitive-include.h
+// RUN: %clang_cc1 -fsyntax-only -Wnonportable-include-path-separator -I%t %s 2>&1 | FileCheck %s
----------------
AaronBallman wrote:

Can you also add a test with `-Wnonportable-include-path -Wno-nonportable-include-path-separator` to show that we correctly silence the diagnostic in that case?

https://github.com/llvm/llvm-project/pull/186770