[flang] move getKindMapping() calls out of FIROpBuilder ctor calls

FirOpBuilder takes a fir::KindMapping reference. When the getKindMapping()
call is made inside the ctor call, the lifetime of this reference may
be as short as the ctor call (at least with when building flang in
release mode with clang 8). This can cause segfaults when later using
the FirOpBuilder.

Ensure the kindMap passed to the FirOpBuilder ctor is the same as the
FirOpBuilder.

Differential Revision: https://reviews.llvm.org/D129494