[DFSan] Add `zeroext` attribute for callbacks with 8bit shadow variable arguments

Add `zeroext` attribute for below callbacks' first parameter
(8bit shadow variable arguments) to conform to many platforms'
ABI calling convention and some compiler behavior.
- __dfsan_load_callback
- __dfsan_store_callback
- __dfsan_cmp_callback
- __dfsan_conditional_callback
- __dfsan_conditional_callback_origin
- __dfsan_reaches_function_callback
- __dfsan_reaches_function_callback_origin

The type of these callbacks' first parameter is u8 (see the
definition of `dfsan_label`). First, many platforms' ABI
requires unsigned integer data types (except unsigned int)
are zero-extended when stored in general-purpose register.
Second, the problem is that compiler optimization may assume
the arguments are zero-extended and, if not, misbehave, e.g.
it uses an `i8` argument to index into a jump table. If the
argument has non-zero high bits, the output executable may
crash at run-time. So we need to add the `zeroext` attribute
when declaring and calling them.

Reviewed By: browneee, MaskRay

Differential Revision: https://reviews.llvm.org/D140689