HLSL: 1) Implement lookahead buffers/stacks for token advance/recede, 2) use it for cast operation.

The grammar now accepts type casts, like "(int)x", but that
has to be disambiguated from "(a + b)", needed deeper lookahead
and backing up than what existed so far.

gtests/Hlsl.FromFile.cpp

@@ -73,6 +73,7 @@ INSTANTIATE_TEST_CASE_P(
     ToSpirv, HlslCompileTest,
     ::testing::ValuesIn(std::vector<FileNameEntryPointPair>{
         {"hlsl.assoc.frag", "PixelShaderFunction"},
+        {"hlsl.cast.frag", "PixelShaderFunction"},
         {"hlsl.float1.frag", "PixelShaderFunction"},
         {"hlsl.float4.frag", "PixelShaderFunction"},
         {"hlsl.intrinsics.frag", "PixelShaderFunction"},