Allow :: as an alias for . in imports

This is a first cut at how imports ought to adopt module selectors.

Author: beccadax

lib/Parse/ParseDecl.cpp

@@ -4693,7 +4693,9 @@ ParserResult<ImportDecl> Parser::parseDeclImport(ParseDeclOptions Flags,
                            /*diagnoseDollarPrefix=*/false,
                            diag::expected_identifier_in_decl, "import"))
       return nullptr;
-    HasNext = consumeIf(tok::period);
+    HasNext = consumeIf(tok::period) ||
+        (Context.LangOpts.EnableExperimentalModuleSelector &&
+         consumeIf(tok::colon_colon));
   } while (HasNext);
 
   // Collect all access path components to an import path.

test/NameLookup/module_selector.swift

@@ -1,10 +1,13 @@
-// RUN: %target-typecheck-verify-swift -module-name main -I %S/Inputs -enable-experimental-module-selector
+// RUN: %target-typecheck-verify-swift -sdk %clang-importer-sdk -module-name main -I %S/Inputs -enable-experimental-module-selector
 
 // Make sure the lack of the experimental flag disables the feature:
-// RUN: not %target-typecheck-verify-swift -module-name main -I %S/Inputs 2>/dev/null
+// RUN: not %target-typecheck-verify-swift -sdk %clang-importer-sdk -module-name main -I %S/Inputs 2>/dev/null
 
 import ModuleSelectorTestingKit
 
+import ctypes::bits   // FIXME: ban using :: with submodules?
+import struct ModuleSelectorTestingKit::A
+
 let magnitude: Never = fatalError()
 
 // Test resolution of main:: using `B`