[stdlib] Revert hasNormalizationBoundaryBefore

This property is too specific in that it forces a particular normalization; let's not expose it this way, but instead in the future with a full normalization API.

stdlib/public/core/StringNormalization.swift

@@ -68,7 +68,29 @@ internal enum _Normalization {
     }
     return length == string.count
   }
+}
 
+extension UnicodeScalar {
+  // Normalization boundary - a place in a string where everything left of the
+  // boundary can be normalized independently from everything right of the
+  // boundary. The concatenation of each result is the same as if the entire
+  // string had been normalized as a whole.
+  //
+  // Normalization segment - a sequence of code units between two normalization
+  // boundaries (without any boundaries in the middle). Note that normalization
+  // segments can, as a process of normalization, expand, contract, and even
+  // produce new sub-segments.
+
+  // Whether this scalar value always has a normalization boundary before it.
+  internal var _hasNormalizationBoundaryBefore: Bool {
+    _sanityCheck(Int32(exactly: self.value) != nil, "top bit shouldn't be set")
+    let value = Int32(bitPattern: self.value)
+    return 0 != __swift_stdlib_unorm2_hasBoundaryBefore(
+      _Normalization._nfcNormalizer, value)
+  }
+}
+
+extension _Normalization {
   // When normalized in NFC, some segments may expand in size (e.g. some non-BMP
   // musical notes). This expansion is capped by the maximum expansion factor of
   // the normal form. For NFC, that is 3x.