[libc++] Avoid creating temporaries in unary expressions involving valarray

Currently, unary expressions involving valarray will create a temporary.
This leads to dangling references in expressions like `-a * b`, because
`-a` is a temporary and the resulting expression will refer to it. This
patch fixes the problem by creating a lazy expression to perform the unary
operation instead of eagerly creating a temporary valarray. This is
permitted by the Standard, which does not specify the exact type of
most expressions involving valarrays.

This is technically an ABI break, however I believe the actual potential
for breakage is very low.

rdar://90152242

Differential Revision: https://reviews.llvm.org/D125019

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