Issue Description:

Description

When declaring protocol conformance using an extension, it is not possible to redeclare that extension, regardless of whether the first declaration was successful or failed due to compilation errors.

This makes it difficult to quickly fix errors by modifying the broken extension and furthermore leaves types in an inconsistent state, where they are reported as being conformant but do not have required members.

Expected results:

When declaring an extension that declares protocol conformance, the extended type should not become conformant to that extension if the extension has compilation errors.

Example:

 $ swift
 Welcome to Apple Swift version 5.5 (swiftlang-1300.0.20.104 clang-1300.0.21.1).
 Type :help for assistance.
 1> struct Foo { 
 2. }

 3> extension Foo: CustomStringConvertible {}
 error: repl.swift:3:1: error: type 'Foo' does not conform to protocol 'CustomStringConvertible'
 extension Foo: CustomStringConvertible {}
 ^

Swift.CustomStringConvertible:2:9: note: protocol requires property 'description' with type 'String'
 var description: String

{ get }

^

3> extension Foo: CustomStringConvertible { 
4.     public var description: String { 
5.         return "Foo" 
6.     }
7. } 
 error: repl.swift:3:16: error: redundant conformance of 'Foo' to protocol 'CustomStringConvertible'
 extension Foo: CustomStringConvertible {
 ^

repl.swift:3:1: note: 'Foo' declares conformance to protocol 'CustomStringConvertible' here
 extension Foo: CustomStringConvertible {}
 ^

 

First, an invalid conformance is declared for CustomStringConvertible (missing description property).

This causes an error to be reported, but Foo is still marked as being conformant to CustomStringConvertible. (Foo() is CustomStringConvertible evaluates to true)

When declaring a function, such as

func bar(_ x: CustomStringConvertible) {
    print(x.description)
}

and calling it with an instance of Foo following the initial, broken conformance declaration, a symbol lookup error is raised:

error: Couldn't lookup symbols:
  protocol conformance descriptor for __lldb_expr_1.Foo : Swift.CustomStringConvertible in __lldb_expr_3