[DirectX][SPIRV] add support for sv_barycentrics

[spall]

add support for sv_barycentrics for both directx and spirv. add two tests for sv_barycentrics, one which is noperspective.

Closes #1449

Assisted by GPT-5.6 Sol

[Icohedron]

I'm seeing this failure shared among multiple CI checks:

• https://github.com/llvm/offload-test-suite/actions/runs/36463082938/job/109066355074?pr=1535#step:14:213
• https://github.com/llvm/offload-test-suite/actions/runs/36463082938/job/109066355248?pr=1535#step:14:218
• https://github.com/llvm/offload-test-suite/actions/runs/36463082938/job/109066355337?pr=1535#step:14:212
• https://github.com/llvm/offload-test-suite/actions/runs/36463082938/job/109076759461?pr=1535#step:13:707
• https://github.com/llvm/offload-test-suite/actions/runs/36463082938/job/109076914413?pr=1535#step:13:722

FAIL: OffloadTest-d3d12 :: Feature/Semantics/Barycentrics.test (561 of 725)
******************** TEST 'OffloadTest-d3d12 :: Feature/Semantics/Barycentrics.test' FAILED ********************
...
# .---command stderr------------
# |   Mismatch at element 0: expected=0.5 (0x1.0000000000000p-1), actual=0.500978 (0x1.00803e0000000p-1)
# |   Mismatch at element 1: expected=0.25 (0x1.0000000000000p-2), actual=0.249511 (0x1.feff7e0000000p-3)
# |   Mismatch at element 2: expected=0.25 (0x1.0000000000000p-2), actual=0.249511 (0x1.feff9c0000000p-3)
# | Test failed: Barycentrics
# | Comparison Rule: BufferFloatULP
# | ULP: 2
# | Expected:
# | ---
# | Name:            Output_Expected
# | Format:          Float32
# | Stride:          12
# | Data:            [ 0.5, 0.25, 0.25 ]
# | OutputProps:
# |   Height:          0
# |   Width:           0
# |   Depth:           0
# | ...
# | Got:
# | ---
# | Name:            Output
# | Format:          Float32
# | Stride:          12
# | Data:            [ 0.500978, 0.249511, 0.249511 ]
# | OutputProps:
# |   Height:          0
# |   Width:           0
# |   Depth:           0
# | ...
# | Full Hex representation of Expected Buffer Values:
# | [ 0x3f000000, 0x3e800000, 0x3e800000 ]
# | Full Hex representation of Actual Buffer Values:
# | [ 0x3f00401f, 0x3e7f7fbf, 0x3e7f7fce ]

They all agree the output should be [ 0.500978, 0.249511, 0.249511 ] but the test expects [ 0.5, 0.25, 0.25 ]