gollvm: sync with trunk at a61fa1a4b9d, part 2

Revise previous change in CL 240684, not all instances of
deleteValue() were being handled properly.

Change-Id: I5400919ee8b2440e6749f8f712ba7f83eeb89735
Reviewed-on: https://go-review.googlesource.com/c/gollvm/+/241068
Reviewed-by: Cherry Zhang <cherryyz@google.com>

bridge/go-llvm.cpp

diff --git a/bridge/go-llvm.cpp b/bridge/go-llvm.cpp
index 630a49a..445259c 100644
--- a/bridge/go-llvm.cpp
+++ b/bridge/go-llvm.cpp
@@ -2192,7 +2192,10 @@
         llvm::cast<llvm::PointerType>(old->getType())->getElementType();
     llvm::Constant *newDecl = module_->getOrInsertGlobal(gname, declTyp);
     old->replaceAllUsesWith(newDecl);
-    old->deleteValue();
+    // NB: previously we had a call to old->deleteValue() here, but this
+    // appears to no longer be supported, and old->destroyConstant()
+    // results in an assert. For now leave the old constant around, just
+    // remove references to it.
     Bvariable *declbv = valueVarMap_[old];
     declbv->setValue(newDecl);
     valueVarMap_.erase(old);