Fix memory leak of MCSymbolData in MCAsmStreamer.

Leak identified by LSan and reported by Kostya Serebryany.

Let's get a bit experimental here... in theory our minimum compiler
versions support unordered_map.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207118 91177308-0d34-0410-b5e6-96231b3b80d8