[HLSL][LongVec] Add Modf long vector support

I predict this will be super easy once the intrinsic is implemented as it should just be adding `let VaryingLongVector = 1;` to `clang/include/clang/Basic/HLSLIntrinsics.td`

Start in clang/include/clang/Basic/HLSLIntrinsics.td and locate the Modf intrinsic definition. Confirm how long-vector support is represented there, then verify that Modf accepts long vectors as intended; the issue does not name a specific test to run.

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