Allow multiple calls to InitLLVM() (NFC)

In e99dee82, the "out_of_memory_new_handler" was changed to be
explicitly initialized instead of relying on a global static
constructor.
However before this change, install_out_of_memory_new_handler could be
called multiple times while it asserts right now.
We can be more tolerant to calling multiple time InitLLVM without
reintroducing a global constructor for this handler.

Differential Revision: https://reviews.llvm.org/D86330