Properties on intersections should be readonly only if all declarations are

Fixes microsoft#45122

Author: bwrrp

src/compiler/checker.ts

@@ -11795,8 +11795,15 @@ namespace ts {
                                 }
                             }
                         }
-                        checkFlags |= (isReadonlySymbol(prop) ? CheckFlags.Readonly : 0) |
-                            (!(modifiers & ModifierFlags.NonPublicAccessibilityModifier) ? CheckFlags.ContainsPublic : 0) |
+                        // For unions, prop is readonly if any definition of it is readonly
+                        if (isUnion && isReadonlySymbol(prop)) {
+                            checkFlags |= CheckFlags.Readonly;
+                        }
+                        // For intersections, prop is writable if any definition of it is writable
+                        else if (!isUnion && !isReadonlySymbol(prop)) {
+                            checkFlags &= ~CheckFlags.Readonly;
+                        }
+                        checkFlags |= (!(modifiers & ModifierFlags.NonPublicAccessibilityModifier) ? CheckFlags.ContainsPublic : 0) |
                             (modifiers & ModifierFlags.Protected ? CheckFlags.ContainsProtected : 0) |
                             (modifiers & ModifierFlags.Private ? CheckFlags.ContainsPrivate : 0) |
                             (modifiers & ModifierFlags.Static ? CheckFlags.ContainsStatic : 0);