[C++20] [Modules] Don't generate the defintition for non-const available...

[C++20] [Modules] Don't generate the defintition for non-const available external variables (#93530)

Close https://github.com/llvm/llvm-project/issues/93497

The root cause of the problem is, we mark the variable from other
modules as constnant in LLVM incorrectly. This patch fixes this problem
by not emitting the defintition for non-const available external
variables. Since the non const available externally variable is not
helpful to the optimization.