[flang]Zero Initialize simple types

Instead of filling uninitialized global variables with "undef",
initialize them with 0. Only for Integer, Float or Logical type
variables. Complex, user defined data structures, arrays, etc
are not supported at this point.

This patch fixes the main problem of
https://github.com/llvm/llvm-project/issues/62432

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D149877