HLSL: allow implicit array sizing.

In HLSL array sizes need not be provided explicitly in all circumstances.
For example, this is valid (note no number between the [ ]):

  // no explicit array size
  uniform float g_array[] = { 1, 2, 3, 4, 5 };

This PR does not attempt to validate most invalid cases.

A new test is added to verify the resulting linker objects.

gtests/Hlsl.FromFile.cpp

@@ -81,6 +81,7 @@ INSTANTIATE_TEST_CASE_P(
     ToSpirv, HlslCompileTest,
     ::testing::ValuesIn(std::vector<FileNameEntryPointPair>{
         {"hlsl.array.frag", "PixelShaderFunction"},
+        {"hlsl.array.implicit-size.frag", "PixelShaderFunction"},
         {"hlsl.assoc.frag", "PixelShaderFunction"},
         {"hlsl.attribute.frag", "PixelShaderFunction"},
         {"hlsl.buffer.frag", "PixelShaderFunction"},