Add test for `fwidth`

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> Note: This test is parented to the matrix implementation issue because the [original implementation](https://github.com/llvm/llvm-project/issues/99120) is closed.

In `Test\Feature\HLSLLib` create, if applicable: (reference the HLSL headers for the types supported by the intrinsic)
- [ ] Test for 16 bit int types (`fwidth.int16.test`)
- [ ] Test for Half type (`fwidth.fp16.test`)
- [ ] Test for 32 bit types (`fwidth.32.test`)
- [ ] Test for 64 bit int types (`fwidth.int64.test`)
- [ ] Test for Double type (`fwidth.fp64.test`)

Matrix tests are also needed:
- [ ] https://github.com/llvm/wg-hlsl/issues/388

A .test file should contain
- [ ] a source portion; if multiple .test files can use the same source, it can go in its own file and be referenced in the run portion of each .test file instead.
```
#--- source.hlsl

StructuredBuffer In : register(t0);
RWStructuredBuffer Out : register(u1);

[numthreads(1,1,1)]
void main() {
// A single test can test for all scalar/vector length as shown here.
Out[0] = length(In[0]); // Test float4
Out[1] = length(In[1].x); // Test float
Out[2] = length(In[1].yzw); // Test float3
Out[3] = length(In[1].yz); // Test float2
}
```
- [ ] a yaml portion; if multiple .test files can use the same YAML, it can go in its own file and be referenced in the run portion of each .test file instead.
```
//--- pipeline.yaml

---
Shaders:
- Stage: Compute
Entry: main
DispatchSize: [1, 1, 1]
Buffers:
- Name: In
Format: Float32
Stride: 16
Data: [ 4, 4, 4, 4, 3.14159, 0, 5, 12 ]
- Name: Out
Format: Float32
Stride: 4
ZeroInitSize: 12
- Name: ExpectedOut # The result we expect
Format: Float32
Stride: 4
Data: [ 8, 3.14159, 13, 5 ]
Results: # A test might have more than 1 result.
- Result: Test1
Rule: BufferFuzzy # there is also a BufferExact rule
ULPT: 1
Actual: Out
Expected: ExpectedOut
DescriptorSets:
- Resources:
- Name: In
Kind: StructuredBuffer
DirectXBinding:
Register: 0
Space: 0
VulkanBinding:
Binding: 0
- Name: Out
Kind: RWStructuredBuffer
DirectXBinding:
Register: 1
Space: 0
VulkanBinding:
Binding: 1
...
#--- end
```
- [ ] A run portion; this will be unique to each .test file.
```
# RUN: split-file %s %t
# RUN: %dxc_target -T cs_6_5 -Fo %t.o %t/source.hlsl
# RUN: %offloader %t/pipeline.yaml %t.o
```
Tests for 16 bit floating point should have a `# REQUIRES: Half`. Also add `-enable-16bit-types` to the RUN line.
Tests for 16 bit integer should have a `# REQUIRES: Int16`. Also add `-enable-16bit-types` to the RUN line.
Tests for 64 bit floating point should have a `# REQUIRES: Double`.
Tests for 64 bit integer should have a `# REQUIRES: Int64`.

Start in Test\Feature\HLSLLib and inspect the HLSL headers to identify the intrinsic's supported types. Add the applicable scalar and vector .test coverage using the source, YAML, and RUN portions shown in the issue, including the required feature flags and options. Run the listed test commands and confirm expected buffer results; matrix coverage is tracked separately in issue 388.

Written by the indexing model from the issue text.