[HLSL] Implement the `ldexp` intrinsic (#138182)
Closes #99133.
Implemented `ldexp` entirely in `hlsl_intrinsics.h` and
`hlsl_intrinsic_helpers.h`, added coresponding tests in
`clang/test/CodeGenHLSL/builtins/ldexp.hlsl` and
`clang/test/SemaHLSL/BuiltIns/ldexp-errors.hlsl`.
