[SPIR-V] invalid OpImageWrite instruction generated if vk::image_format is not specified.

[Keenuts]

Issue derived from (#4772)

HLSL has attributes to specify the texture format (https://github.com/microsoft/DirectXShaderCompiler/blob/main/docs/SPIR-V.rst#textures).

When not specified, the fallback is Rgba32f (first type of the list).
Given this sample:

RWTexture2D<float3> uavs[2];

[numthreads(1,1,1)]
void maincomp(uint3 threadID : SV_DispatchThreadID)
{
  uavs[threadID.z][threadID.xy] = float3(0.5, 0.2, 1.0);
}

and this command:

dxc repro.hlsl -E maincomp -T cs_6_0 -spirv -O0

The generated code is invalid (breaks VUID-RuntimeSpirv-OpImageWrite-07112)

OpImageWrite to any Image whose Image Format is not Unknown must have the Texel operand contain at least as many components as the corresponding [VkFormat](https://registry.khronos.org/vulkan/specs/1.3-extensions/man/html/VkFormat.html) as given in the [SPIR-V Image Format compatibility table](https://registry.khronos.org/vulkan/specs/1.3-extensions/html/vkspec.html#spirvenv-image-formats)

Reason is we default the image format to RGBA32F instead of unknown.
A quick inspection in DXC showed we set it to unknown at first, but then the information is ignored when the type is lowered, and the image format is selected depending on the sampled type (float3), hence RGBA32F, since RGB32F doesn't exists.

Since we emit programs with the shader capability, we could enable StorageImageReadWithoutFormat and keep the format as unknown.