Fix backwards count of Indic graphemes

Author: Azoy

stdlib/public/core/StringGraphemeBreaking.swift

@@ -390,7 +390,8 @@ extension _StringGuts {
       // If we're currently in an indic sequence (or if our lhs is a linking
       // consonant), then this check and everything underneath ensures that
       // we continue being in one and may check if this extend is a Virama.
-      if state.isInIndicSequence || scalar1._isLinkingConsonant {
+      if state.isInIndicSequence ||
+         (!isBackwards && scalar1._isLinkingConsonant) {
         if y == .extend {
           let extendNormData = Unicode._NormData(scalar2, fastUpperbound: 0x300)
 

validation-test/stdlib/String.swift

@@ -2333,6 +2333,13 @@ if #available(SwiftStdlib 5.6, *) {
     let test10 = "\u{003F}\u{094D}\u{0924}" // 2
     expectEqual(2, test10.count)
     expectBidirectionalCount(2, test10)
+
+#if _runtime(_ObjC)
+    let test11Foreign = NSString(string: "\u{930}\u{93e}\u{91c}\u{94d}") // 2
+    let test11 = test11Foreign as String
+    expectEqual(2, test11.count)
+    expectBidirectionalCount(2, test11)
+#endif
   }
 }