[stdlib] Don’t use assert() in the stdlib

assert() is designed to be used in user code only; the equivalent stdlib function is called _internalInvariant().

rdar://57101013

stdlib/public/core/StringNormalization.swift

@@ -108,7 +108,7 @@ extension UnsafeBufferPointer where Element == UInt8 {
     if index == 0 || index == count {
       return true
     }
-    assert(!UTF8.isContinuation(self[_unchecked: index]))
+    _internalInvariant(!UTF8.isContinuation(self[_unchecked: index]))
 
     // Sub-300 latiny fast-path
     if self[_unchecked: index] < 0xCC { return true }