[flang] Restore stack after allocas created by TargetRewrite.

This resolves issues with running out of stack on examples like
https://fortran-lang.discourse.group/t/modern-fortran-sample-code/2019/18
reported by @clementval.

When target rewrite creates alloca(s) around a call, we need to insert
stacksave/stackrestore to free the allocated stack. Better performant
code may be achieved by placing the alloca(s) outside of loops,
but the placement has to behave correctly with regards to OpenMP/OpenACC/etc.
dialect operations that have special representation for "private"
objects. This is a concervative fix for correctness issue.

Differential Revision: https://reviews.llvm.org/D149222