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

Description

[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

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