ASTGen: account for alternate path separators

Windows supports `\` and `/` as path separators.  Account for both in
computing the basename.  This repairs a few test failures on Windows.

Author: compnerd

lib/ASTGen/Sources/ASTGen/SourceManager+MacroExpansionContext.swift

@@ -43,7 +43,7 @@ extension String {
   /// Retrieve the base name of a string that represents a path, removing the
   /// directory.
   var basename: String {
-    guard let lastSlash = lastIndex(of: "/") else {
+    guard let lastSlash = lastIndex(where: { ["/", "\\"].contains($0) }) else {
       return self
     }