[Module-scope lookup] Only add macro-expanded names that match.

When we look into expanded macros to find declarations for module-scope
name lookup, make sure that we only return declarations whose name
matches the requested name.

We were effectively spamming the results with unrelated names, and
most clients trust the results of this lookup (vs. filtering them
further), so they would get confused. The particular failure for this
test is that type reconstruction would fail because we were looking
for one unique name, but we found many non-matching names, and type
reconstruction bails out rather than try to filter further.

(cherry picked from commit 8e63bf1)

Author: DougGregor

lib/AST/Module.cpp

@@ -444,10 +444,12 @@ void SourceLookupCache::lookupValue(DeclName Name, NLKind LookupKind,
     return;
 
   for (auto *unexpandedDecl : auxDecls->second) {
-    // Add expanded peers to the result.
+    // Add expanded peers and freestanding declarations to the result.
     unexpandedDecl->forEachMacroExpandedDecl(
         [&](ValueDecl *decl) {
-          Result.push_back(decl);
+          if (decl->getName().matchesRef(Name)) {
+            Result.push_back(decl);
+          }
         });
   }
 }

test/Macros/macro_expand_peers.swift

@@ -13,7 +13,7 @@
 // RUN: %target-swift-frontend -swift-version 5 -typecheck -load-plugin-library %t/%target-library-name(MacroDefinition) -parse-as-library %s -disable-availability-checking -dump-macro-expansions > %t/expansions-dump.txt 2>&1
 // RUN: %FileCheck -check-prefix=CHECK-DUMP %s < %t/expansions-dump.txt
 
-// RUN: %target-build-swift -swift-version 5 -Xfrontend -disable-availability-checking -load-plugin-library %t/%target-library-name(MacroDefinition) -parse-as-library %s -o %t/main -module-name MacroUser
+// RUN: %target-build-swift -swift-version 5 -Xfrontend -disable-availability-checking -load-plugin-library %t/%target-library-name(MacroDefinition) -parse-as-library %s -o %t/main -module-name MacroUser -g
 // RUN: %target-codesign %t/main
 // RUN: %target-run %t/main | %FileCheck %s -check-prefix=CHECK-EXEC