[IR] Export ConstantFold.h header (NFC)

This is in preparation for https://discourse.llvm.org/t/rfc-remove-most-constant-expressions/63179.
As part of that change, we'll want to invoke some of these constant
folding APIs explicitly, as it won't happen as part of
ConstantExpr::getXYZ() anymore.

Ideally, we'd merge these with the DL-aware constant folding APIs
and only call those, but this is not easily possible for some
current usages (most important IRBuilder, which uses DL-independent
constant folding by default, and some major layering changes would
be needed to change that).

This is basically a reboot of D115035 with different motivation.

Differential Revision: https://reviews.llvm.org/D128213