AST: Add scoped import shadowing rule to removeShadowedDecls()

This simulates the shadowing done by ModuleNameLookup, which is about to be
removed.

The basic idea is that top-level declarations found via scoped imports
take precedence over unscoped imports.

Author: slavapestov

lib/AST/NameLookup.cpp

@@ -206,6 +206,17 @@ static void recordShadowedDeclsAfterSignatureMatch(
       if (firstModule != secondModule &&
           firstDecl->getDeclContext()->isModuleScopeContext() &&
           secondDecl->getDeclContext()->isModuleScopeContext()) {
+        // First, scoped imports shadow unscoped imports.
+        bool firstScoped = imports.isScopedImport(firstModule, name, dc);
+        bool secondScoped = imports.isScopedImport(secondModule, name, dc);
+        if (!firstScoped && secondScoped) {
+          shadowed.insert(firstDecl);
+          break;
+        } else if (firstScoped && !secondScoped) {
+          shadowed.insert(secondDecl);
+          continue;
+        }
+
         // Now check if one module shadows the other.
         if (imports.isShadowedBy(firstModule, secondModule, name, dc)) {
           shadowed.insert(firstDecl);

test/NameBinding/import-resolution.swift

@@ -32,8 +32,8 @@ letters.asdf.A // expected-error{{module 'letters' has no member named 'asdf'}}
 var uA : A // expected-error {{'A' is ambiguous for type lookup in this context}}
 var uB : B = abcde.B()
 var uC : C // expected-error {{'C' is ambiguous for type lookup in this context}}
-var uD : D // expected-error {{'D' is ambiguous for type lookup in this context}}
-var uE : E // expected-error {{'E' is ambiguous for type lookup in this context}}
+var uD : D = asdf.D()
+var uE : E = aeiou.E()
 var uF : F = letters.F()
 
 var qA1 : abcde.A // okay