[flang] Set KIND in compiler generated COUNT for SIZE(PACK) (#79801)

Compiler was rewriting SIZE(PACK(x, MASK)) to COUNT(MASK). It was
wrapping the COUNT call without a KIND argument (leading to INTEGER(4)
result in the characteristics) in an Expr<ExtentType> (implying
INTEGER(8) result), this lead to inconsistencies that later hit verifier
errors in lowering.

Set the KIND argument to the KIND of ExtentType to ensure the built
expression is consistent.

This requires giving access to some safe place where the "kind" name can
be saved and turned into a CharBlock (count has a DIM argument that
require using the KIND keyword here). For the FoldingContext that belong
to SemanticsContext, this is the same string set as the one used by
SemanticsContext for similar purposes.