[DirectX] Disable all libcalls for DXIL in TargetLibraryInfo.cpp (#138991)
Fixes #138787
To the best of my knowledge, DXIL does not (and should not) support any
of the libcalls in
[`TargetLibraryInfo.def`](https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/Analysis/TargetLibraryInfo.def).
Math libcalls are not used in HLSL and also do not have lowerings to
DXIL. (The current implementation of math functions are done via
intrinsics.)
If there is a mistake with disabling all libcalls, then the libcalls we
need can be re-enabled in a follow-up PR.
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Co-authored-by: Justin Bogner <mail@justinbogner.com>