[ISan] Fix issue with constexpr.

[ISan] Fix issue with constexpr.

The semantic of the constexpr does not allow us
to convert POINTER type to INTERGER type inside constexpr.
All ISAN instrumentation should be folded, but it's
little bit tricky, in case we should convert INTEGER
type back to POINTER type, because INTEGER type could
not be assigned to POINTER type.
Another issue is that some actual value of PARM_DECL
could not be getting by "constexpr context" and
could not be folded.
So, just disable ISAN insrumentation and pointer
conversions for constexpr semantic. In this case we
can rely on current_function_decl while processing function,
but also should handle constexpr with pointers:
constexpr unsigned *pointer = ptr + index;

Change-Id: I68eb660438a1acd881207219c3d72045c445dcb7