Add r230655 back with a fix.

The issue is that now we have a diag handler during optimizations
and get forward every optimization remark, flooding stdout.

The same filtering should probably be done with or without a
custom handler, but for now just ignore remarks.

Original message:

gold-plugin: "Upgrade" debug info and handle its warnings.

The gold plugin never calls MaterializeModule, so any old debug info
was not deleted and could cause crashes.

Now that it is being "upgraded", the plugin also has to handle warnings
and create Modules with a nice id (it shows in the warning).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230991 91177308-0d34-0410-b5e6-96231b3b80d8

test/tools/gold/drop-debug.ll

@@ -0,0 +1,8 @@
+; RUN: %gold -plugin %llvmshlibdir/LLVMgold.so \
+; RUN:    --plugin-opt=emit-llvm -shared %p/Inputs/drop-debug.bc \
+; RUN:    -o t2.bc 2>&1 | FileCheck %s
+
+; drop-debug.bc was created from "void f(void) {}" with clang 3.5 and
+; -gline-tables-only, so it contains old debug info.
+
+; CHECK: warning: LLVM gold plugin: ignoring debug info with an invalid version (1) in {{.*}}/Inputs/drop-debug.bc