HLSL: add subpass input types and methods

Add support for Subpass Input proposal of issue #1069.

Subpass input types are given as:

    layout(input_attachment_index = 1) SubpassInput<float4> subpass_f;
    layout(input_attachment_index = 2) SubpassInput<int4>   subpass_i;
    layout(input_attachment_index = 3) SubpassInput<uint4>  subpass_u;

    layout(input_attachment_index = 1) SubpassInputMS<float4> subpass_ms_f;
    layout(input_attachment_index = 2) SubpassInputMS<int4>   subpass_ms_i;
    layout(input_attachment_index = 3) SubpassInputMS<uint4>  subpass_ms_u;

The input attachment may also be specified using attribute syntax:

    [[vk::input_attachment_index(7)]] SubpassInput subpass_2;

The template type may be a shorter-than-vec4 vector, but currently user
structs are not supported.  (An unimplemented error will be issued).

The load operations are methods on objects of the above type:

    float4 result = subpass_f.SubpassLoad();
    int4   result = subpass_i.SubpassLoad();
    uint4  result = subpass_u.SubpassLoad();

    float4 result = subpass_ms_f.SubpassLoad(samp);
    int4   result = subpass_ms_i.SubpassLoad(samp);
    uint4  result = subpass_ms_u.SubpassLoad(samp);

Additionally, the AST printer could not print EOpSubpass* nodes.  Now it can.

Fixes #1069

hlsl/hlslScanContext.cpp

@@ -336,6 +336,8 @@ void HlslScanContext::fillInKeywordMap()
     (*KeywordMap)["RWTexture2DArray"] =        EHTokRWTexture2darray;
     (*KeywordMap)["RWTexture3D"] =             EHTokRWTexture3d;
     (*KeywordMap)["RWBuffer"] =                EHTokRWBuffer;
+    (*KeywordMap)["SubpassInput"] =            EHTokSubpassInput;
+    (*KeywordMap)["SubpassInputMS"] =          EHTokSubpassInputMS;
 
     (*KeywordMap)["AppendStructuredBuffer"] =  EHTokAppendStructuredBuffer;
     (*KeywordMap)["ByteAddressBuffer"] =       EHTokByteAddressBuffer;
@@ -827,6 +829,8 @@ EHlslTokenClass HlslScanContext::tokenizeIdentifier()
     case EHTokRWByteAddressBuffer:
     case EHTokRWStructuredBuffer:
     case EHTokStructuredBuffer:
+    case EHTokSubpassInput:
+    case EHTokSubpassInputMS:
         return keyword;
 
     // variable, user type, ...