[HLSL] Implement Texture2D default template (#184207)

The Texture2D type has a default template of float4. This can be written
in a couple way: `Texture2D<>` or `Texture2D`. This must be implemented
for consistenty with DXC in HLSL202x.

To implement `Texture2D<>` we simply add a default type for the template
parameter.

To implement `Texture2D`, we have to add a special case for a template
type without a template instantiation. For HLSL, we check if it is a
texture type. If so, the default type is filled in.

Note that HLSL202x does not support C++17 Class Template Argument
Deduction, so we cannot use that feature to give us `Texture2D`.

See https://github.com/llvm/wg-hlsl/pull/386 for alternatives that were
considered.

Part of #175630.

Assisted-by: Gemini

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