[Type checker] Suppress near-miss warnings within the type definition.

There are a number of things that can only be written within the main
type definition (e.g., required initializers, stored properties),
making it far more likely that we'll get false positives from the
newly-introduced "near-miss" warnings for protocol
conformances. Moreover, sometimes a number of protocol conformances
are placed on the main type definition, increasing the potential for
false positives.

Suppress near-miss warnings for potential candidates when the protocol
conformance is stated on the main type definition itself. Therefore,
we only perform near-miss checking of non-@objc requirements when the
conformance itself was declared on an extension, which is roughly the
development style that near-miss warnings favor.

Fixes rdar://problem/37283860.

(cherry picked from commit 3883ff0)

Author: DougGregor

lib/Sema/TypeCheckProtocol.cpp

@@ -4021,6 +4021,11 @@ static bool shouldWarnAboutPotentialWitness(
       isUnlabeledInitializerOrSubscript(witness))
     return false;
 
+  // For non-@objc requirements, only warn if the witness comes from an
+  // extension.
+  if (!req->isObjC() && !isa<ExtensionDecl>(witness->getDeclContext()))
+    return false;
+
   // If the score is relatively high, don't warn: this is probably
   // unrelated.  Allow about one typo for every four properly-typed
   // characters, which prevents completely-wacky suggestions in many

test/decl/ext/protocol.swift

@@ -526,7 +526,7 @@ struct SConforms7a : PConforms7 { }
 protocol PConforms8 {
   associatedtype Assoc
 
-  func method() -> Assoc // expected-note{{requirement 'method()' declared here}}
+  func method() -> Assoc
   var property: Assoc { get }
   subscript (i: Assoc) -> Assoc { get }
 }
@@ -551,10 +551,7 @@ func testSConforms8b() {
 }
 
 struct SConforms8c : PConforms8 { 
-  func method() -> String { return "" } // expected-warning{{instance method 'method()' nearly matches defaulted requirement 'method()' of protocol 'PConforms8'}}
-  // expected-note@-1{{candidate has non-matching type '() -> String' [with Assoc = Int]}}
-  // expected-note@-2{{move 'method()' to an extension to silence this warning}}
-  // expected-note@-3{{make 'method()' private to silence this warning}}
+  func method() -> String { return "" } // no warning in type definition
 }
 
 func testSConforms8c() {