Until C++20, std::vector doesn't have a constexpr constructor, which
means TType cannot use a `TVector` for `mArraySizes` if an arrayed type
needs to be created constexpr.  This is needed for the upcoming
textureGatherOffsets implementation.

A new TSpan class is introduced, based on std::span (from C++20) that
holds the pointer/size allocated from a TVector without owning it.
Since TVector's allocation are made from a pool, the allocated memory
will live beyond the vector's destruction.  `TType::mArraySizes` is
changed to this type.

This change will allow a new constexpr constructor to be added to TType
that takes a TSpan as array directly, a value which is constexpr
initialized from a static array (instead of TVector).

Bug: angleproject:3569
Change-Id: I78793b0f4c64519e0ebe30cf6e0de995ba70035d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1968260Reviewed-by: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>