[libomptarget] Fix a bug whereby firstprivates are not copied over to the device

The check for the TO flag when processing firstprivates is missing. As a result,
sometimes the device copy of a firstprivate never gets initialized. Currectly we
try to force lambda structs to be allocated immediately by marking them as a
non-firstprivate, so that PrivateArgumentManagerTy::addArg allocates memory for
them immediately. However, calling addArg with IsFirstPrivate=false makes the
function skip initializing the device copy. Whether an argument is firstprivate
and whether we need to allocate memory immediately are not synonyms, so this
patch introduces one more control variable for immediate allocation and sets it
apart from initialization.

Differential Revision: https://reviews.llvm.org/D102890