Intrinsics: expand semantics of LLVMExtendedVectorType (& trunc)

These are used in the ARM backends to aid type-checking on patterns involving
intrinsics. By making sure one argument is an extended/truncated version of
another.

However, there's no reason to limit them to just vectors types. For example
AArch64 has the instruction "uqshrn sD, dN, #imm" which would naturally use an
intrinsic taking an i64 and returning an i32.
[sanitizer] Intercept fgetpwent / fgetgrent.

These interceptors require deep unpoisoning of return values.
While at it, we do the same for all other pw/gr interceptors to
reduce dependency on libc implementation details.