[SanitizerCoverage] Fix wrong pointer type return from CreateSecStartEnd()

`CreateSecStartEnd()` will return pointer to the input type, so when called with `CreateSecStartEnd(M, SanCovCFsSectionName, IntptrPtrTy)`, `SecStartEnd.first` and `SecStartEnd.second` will have type `IntptrPtrPtrTy`, not `IntptrPtrTy`.

This problem should not impact the functionality and with opaque pointer enable, this will not trigger any alarm. But if runs with `-no-opaque-pointers`, this mismatch pointer type will cause type check assertion in `CallInst::init()` to fail.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D137310