Allow multiple calls to InitLLVM() (NFC)

[mehdi_amini]

Summary

In e99dee82b0, 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.

[MaskRay]

LGTM.

llvm/lib/Support/ErrorHandling.cpp

static void out_of_memory_new_handler() {
   llvm::report_bad_alloc_error("Allocation failed");
 }
 
 // Installs new handler that causes crash on allocation failure. It is called by
 // InitLLVM.
 void llvm::install_out_of_memory_new_handler() {
   std::new_handler old = std::set_new_handler(out_of_memory_new_handler);
   (void)old;
-  assert(old == nullptr && "new-handler already installed");
+  assert((old == nullptr || old == out_of_memory_new_handler) &&
+         "new-handler already installed");
 }
 #endif
 
 void llvm::llvm_unreachable_internal(const char *msg, const char *file,
                                      unsigned line) {
   // This code intentionally doesn't call the ErrorHandler callback, because
   // llvm_unreachable is intended to be used to indicate "impossible"
   // situations, and not legitimate runtime errors.