[flang] Store KindMapping by value in FirOpBuilder

Previously only a constant reference was stored in the FirOpBuilder.
However, a lot of code was merged using

FirOpBuilder builder{rewriter, getKindMapping(mod)};

This is incorrect because the KindMapping returned will go out of scope
as soon as FirOpBuilder's constructor had run. This led to an infinite
loop running some tests using HLFIR (because the stack space containing
the kind mapping was re-used and corrupted).

One solution would have just been to fix the incorrect call sites,
however, as a large number of these had already made it past review, I
decided to instead change FirOpBuilder to store its own copy of the
KindMapping. This is not costly because nearly every time we construct a
KindMapping is exclusively to construct a FirOpBuilder. To make this
common pattern simpler, I added a new constructor to FirOpBuilder which
calls getKindMapping().

Differential Revision: https://reviews.llvm.org/D151881

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