[Libomptarget] Replace Value RAII with default value

This patch replaces the ValueRAII pointer with a default 'nullptr'
value. Previously this was initialized as a reference to an existing
variable. The use of this variable caused overhead as the compiler could
not look through the uses and determine that it was unused if 'Active'
was not set. Because of this accesses to the variable would be left in
the runtime once compiled.

Fixes #53641

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D119187