Plug dlerror() leak for swift_demangle

Summary:
InitializeSwiftDemangler() attempts to resolve the
swift_demangle symbol. If this is not available, we
observe dlerror message leak.

Caught on NetBSD/amd64 in TSan.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, kubamracek, vitalybuka, dvyukov, eugenis

Reviewed By: dvyukov

Subscribers: llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D40382

llvm-svn: 318980