Platform: add HLSL compiler to the WinSDK modulemap

The DirectX subsystem may require access to the HLSL compiler for
building the shaders before uploading to the GPU.  This is adds to the
modulemap the D3DCompiler module to get access to the compiler.

(cherry picked from commit 997cb0f)

Author: compnerd

stdlib/public/Platform/winsdk.modulemap

@@ -165,9 +165,6 @@ module WinSDK [system] {
       header "d3d11_2.h"
       header "d3d11_3.h"
       header "d3d11_4.h"
-
-      header "../shared/dxgi1_6.h"
-
       export *
 
       link "d3d11.lib"
@@ -182,6 +179,23 @@ module WinSDK [system] {
       link "dxgi.lib"
     }
 
+    // FIXME(compnerd) DXGI is part of the Direct3D interfaces currently; we
+    // should split it out, but because it is part of the D3D11 interfaces, this
+    // separate module is meant to augment the uncovered portions only.
+    module _DXGI {
+      header "../shared/dxgi1_6.h"
+      export *
+
+      link "dxgi.lib"
+    }
+
+    module D3DCompiler {
+      header "d3dcompiler.h"
+      export *
+
+      link "d3dcompiler.lib"
+    }
+
     module XAudio29 {
       header "xaudio2.h"
       header "xaudio2fx.h"