Sema: Tweak re-declaration checking behavior to match old logic

The parser's own re-declaration checking for local declarations has
slightly different behavior than Sema's re-declaration check.

Whereas Sema rejects this:

class C {
  let x = 123
  func x() {}
}

The parser accepts this:

func f() {
  let x = 123
  func x() {}
}

With Sema's check now handling both cases, fudge the behavior to
match the parser in the local case.

Author: slavapestov

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -572,6 +572,11 @@ CheckRedeclarationRequest::evaluate(Evaluator &eval, ValueDecl *current) const {
     if (currentDC->isTypeContext() != other->getDeclContext()->isTypeContext())
       continue;
 
+    // In local context, only consider exact name matches.
+    if (currentDC->isLocalContext() &&
+        current->getName() != other->getName())
+      continue;
+
     // Check whether the overload signatures conflict (ignoring the type for
     // now).
     auto otherSig = other->getOverloadSignature();

test/Sema/redeclaration-checking.swift

@@ -93,4 +93,9 @@ func stmtTest() {
   // expected-error@-2 {{invalid redeclaration of 'x'}}
   // expected-warning@-3 2{{never used}}
   // expected-warning@-4 {{unreachable}}
+}
+
+func fullNameTest() {
+  let x = 123 // expected-warning {{never used}}
+  func x() {}
 }