jit: Fix type used for Datum values in LLVM IR.

jit: Fix type used for Datum values in LLVM IR.

Commit 2a600a93 made Datum 8 bytes wide everywhere.  It was no longer
appropriate to use TypeSizeT on 32 bit systems, and JIT compilation
would fail with various type check errors.  Introduce a separate
LLVMTypeRef with the name TypeDatum.  TypeSizeT is still used in some
places for actual size_t values.

Reported-by: Dmitry Mityugov <d.mityugov@postgrespro.ru>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Tested-by: Dmitry Mityugov <d.mityugov@postgrespro.ru>
Discussion: https://postgr.es/m/0a9f0be59171c2e8f1b3bc10f4fcf267%40postgrespro.ru