Fix withCString in the presence of old binaries, and optimize it for ASCII

Author: Catfish-Man

stdlib/public/core/String.swift

@@ -723,7 +723,8 @@ extension String {
     encodedAs targetEncoding: TargetEncoding.Type,
     _ body: (UnsafePointer<TargetEncoding.CodeUnit>) throws -> Result
   ) rethrows -> Result {
-    if targetEncoding == UTF8.self {
+    if targetEncoding == UTF8.self ||
+       (targetEncoding == Unicode.ASCII.self && _guts.isASCII) {
       return try unsafe self.withCString {
         (cPtr: UnsafePointer<CChar>) -> Result  in
         _internalInvariant(UInt8.self == TargetEncoding.CodeUnit.self)

stdlib/public/core/StringGuts.swift

@@ -245,6 +245,12 @@ extension _StringGuts {
   internal func _slowWithCString<Result>(
     _ body: (UnsafePointer<Int8>) throws -> Result
   ) rethrows -> Result {
+    if _object.isFastZeroTerminated {
+      // This branch looks unreachable, but can be reached via `withCString`
+      // in binaries that haven't been recompiled since the termination flag
+      // was added to _StringObject. Retry the fast path if so.
+      return try withCString(body)
+    }
     _internalInvariant(!_object.isFastZeroTerminated)
     return try String(self).utf8CString.withUnsafeBufferPointer {
       let ptr = unsafe $0.baseAddress._unsafelyUnwrappedUnchecked