Add a test importing a Clang module with non-identifier characters.

This requires no additional work beyond just parsing the identifier because Clang module maps already support such names in quotes.

Author: allevato

test/ClangImporter/Inputs/custom-modules/WeirdCModule.h

@@ -0,0 +1,3 @@
+typedef struct {
+  int value;
+} S;

test/ClangImporter/Inputs/custom-modules/module.modulemap

@@ -271,3 +271,7 @@ module CommonName {
   header "CommonName.h"
   export *
 }
+
+module "Weird C Module" {
+  header "WeirdCModule.h"
+}

test/ClangImporter/escaped-identifier-module-name.swift

@@ -0,0 +1,6 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -I %S/Inputs/custom-modules %s -verify
+
+import `Weird C Module`
+
+_ = S().value
+_ = `Weird C Module`.S().value