[IR] Add memory attributes for init.trampoline

Based on my reading of https://llvm.org/docs/LangRef.html#llvm-init-trampoline-intrinsic,
init.trampoline writes to the first argument, while the other two
are readnone. These two arguments are only captured and written
into the trampoline memory. This also matches what I see in the
X86TargetLowering::LowerINIT_TRAMPOLINE() implementation.

Differential Revision: https://reviews.llvm.org/D116149