[GC][NFC] Move GCStrategy from CodeGen to IR

We want it to be available in analyzes so that we could use the
CodeGen notion in middle-end passes (for example, to check if
a GC may free some particular pointer).

This is a preparatory patch that simply moves the files around.

Note: if this causes some build issues, this patch must just be reverted.

Differential Revision: https://reviews.llvm.org/D100557
Reviewed By: reames