[clang] Add early exit when checking for const init of arrays.

Before this commit, on code like:

  struct S { ... };
  S arr[10000000];

while checking if arr is constexpr, clang would reserve memory for
arr before running constructor for S. If S turned out to not have a
valid constexpr c-tor, clang would still try to initialize each element
(and, in case the c-tor was trivial, even skipping the constexpr step
limit), only to discard that whole APValue later, since the first
element generated a diagnostic.

With this change, we start by allocating just 1 element in the array to
try out the c-tor and take an early exit if any diagnostics are
generated, avoiding possibly large memory allocation and a lot of work
initializing to-be-discarded APValues.

Fixes 51712 and 51843.

In the future we may want to be smarter about large possibly-constexrp
arrays and maybe make the allocation lazy.

Differential Revision: https://reviews.llvm.org/D113120

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