Stop exporting winrt::impl::get_marshaler to workaround MSVC modules bug (#1592)

* Stop exporting winrt::impl::get_marshaler to workaround MSVC modules bug

* Include base.h in marshal.cpp to verify that include-then-import still works

Author: DefaultRyan

strings/base_marshaler.h

@@ -1,5 +1,5 @@
 
-WINRT_EXPORT namespace winrt::impl
+namespace winrt::impl
 {
     inline std::int32_t make_marshaler(unknown_abi* outer, void** result) noexcept
     {

test/test_cpp20_module/marshal.cpp

@@ -0,0 +1,25 @@
+#include "pch.h"
+#include <objidl.h>
+#include <winrt/base.h>
+
+import std;
+import winrt.Windows.Foundation.Collections;
+
+using namespace winrt;
+
+struct S : implements<S, Windows::Foundation::IStringable>
+{
+    hstring ToString()
+    {
+        return L"S";
+    }
+};
+
+// When winrt::impl::get_marshaler was being exported, an MSVC bug caused the marshaler
+// object to have a null vtable, which caused a crash when calling any method on the marshaler.
+// This test ensures that the marshaler vtable is properly initialized.
+TEST_CASE("IMarshal")
+{
+    auto s = make<S>();
+    auto marshal = s.as<IMarshal>();
+}

test/test_cpp20_module/test_cpp20_module.vcxproj

@@ -25,7 +25,8 @@
       <Configuration>Release</Configuration>
       <Platform>x64</Platform>
     </ProjectConfiguration>
-  </ItemGroup>  <PropertyGroup Label="Globals">
+  </ItemGroup>
+  <PropertyGroup Label="Globals">
     <VCProjectVersion>16.0</VCProjectVersion>
     <ProjectGuid>{B8E3A5CE-4E91-4F27-9B02-E0CAF7E10D72}</ProjectGuid>
     <RootNamespace>test_cpp20_module</RootNamespace>
@@ -91,6 +92,7 @@
     <ClInclude Include="pch.h" />
   </ItemGroup>
   <ItemGroup>
+    <ClCompile Include="marshal.cpp" />
     <ClCompile Include="pch.cpp">
       <PrecompiledHeader>Create</PrecompiledHeader>
     </ClCompile>