[Flang] Lower the repeat intrinsic

The repeat intrinsic creates ncopies of a string. The lowering is to
a runtime call to a function in the flang library. The runtime allocates
the buffer to store the result string. This buffer is freed by code
added in the lowering.

This is part of the upstreaming effort from the fir-dev branch in [1].
[1] https://github.com/flang-compiler/f18-llvm-project

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D121880



Co-authored-by: Valentin Clement <clementval@gmail.com>
Co-authored-by: Jean Perier <jperier@nvidia.com>