[APSInt][OpenMP] Fix isNegative, etc. for unsigned types

Without this patch, APSInt inherits APInt::isNegative, which merely
checks the sign bit without regard to whether the type is actually
signed.  isNonNegative and isStrictlyPositive call isNegative and so
are also affected.

This patch adjusts APSInt to override isNegative, isNonNegative, and
isStrictlyPositive with implementations that consider whether the type
is signed.

A large set of Clang OpenMP tests are affected.  Without this patch,
these tests assume that `true` is not a valid argument for clauses
like `collapse`.  Indeed, `true` fails APInt::isStrictlyPositive but
not APSInt::isStrictlyPositive.  This patch adjusts those tests to
assume `true` should be accepted.

This patch also adds tests revealing various other similar fixes due
to APSInt::isNegative calls in Clang's ExprConstant.cpp and
SemaExpr.cpp: `++` and `--` overflow in `constexpr`, evaluated object
size based on `alloc_size`, `<<` and `>>` shift count validation, and
OpenMP array section validation.

Reviewed By: lebedev.ri, ABataev, hfinkel

Differential Revision: https://reviews.llvm.org/D59712

llvm-svn: 359012