[stdlib] Switch to using unchecked buffer subscript in low-level Unicode helpers

We expect indices to be already validated by the time these are
called, and UBP’s debug-mode checks are compiled into opaque parts
of the stdlib.

(The big exception is that Swift <5.7 used to not do proper index
validation and allowed out-of-bounds accesses. We emulate this
behavior for code that was compiled with such a release, and it turns
out that these UnsafeBufferPointer checks are interfering with the
intended (undefined) behavior.)

rdar://93707276

Author: lorentey

stdlib/public/core/UnicodeHelpers.swift

@@ -64,7 +64,7 @@ internal func _decodeUTF8(
 internal func _decodeScalar(
   _ utf16: UnsafeBufferPointer<UInt16>, startingAt i: Int
 ) -> (Unicode.Scalar, scalarLength: Int) {
-  let high = utf16[i]
+  let high = utf16[_unchecked: i]
   if i + 1 >= utf16.count {
     _internalInvariant(!UTF16.isLeadSurrogate(high))
     _internalInvariant(!UTF16.isTrailSurrogate(high))
@@ -76,7 +76,7 @@ internal func _decodeScalar(
     return (Unicode.Scalar(_unchecked: UInt32(high)), 1)
   }
 
-  let low = utf16[i+1]
+  let low = utf16[_unchecked: i+1]
   _internalInvariant(UTF16.isLeadSurrogate(high))
   _internalInvariant(UTF16.isTrailSurrogate(low))
   return (UTF16._decodeSurrogates(high, low), 2)
@@ -207,7 +207,7 @@ extension _StringGuts {
   @inlinable
   internal func fastUTF8ScalarLength(startingAt i: Int) -> Int {
     _internalInvariant(isFastUTF8)
-    let len = _utf8ScalarLength(self.withFastUTF8 { $0[i] })
+    let len = _utf8ScalarLength(self.withFastUTF8 { $0[_unchecked: i] })
     _internalInvariant((1...4) ~= len)
     return len
   }