Add option -fkeep-persistent-storage-variables to emit all variables that have...

Add option -fkeep-persistent-storage-variables to emit all variables that have a persistent storage duration

This patch adds a new option -fkeep-persistent-storage-variables to emit
all variables that have a persistent storage duration, including global,
static and thread-local variables. This could be useful in cases where
the presence of all these variables as symbols in the object file are
required, so that they can be directly addressed.

Reviewed By: hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D150221