[fir] Update fir.alloca op

Add pinned attributes and speicifc builders.

The pinned attribute helps mark those allocas in OpenMP regions that should not
be hoisted out by an alloca hoisting pass.

This patch is part of the upstreaming effort from fir-dev branch.

Coming from PR: https://github.com/flang-compiler/f18-llvm-project/pull/1065

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D110815



Co-authored-by: Valentin Clement <clementval@gmail.com>