Fix memory leak of MCSymbolData in MCAsmStreamer.
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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@207118 91177308-0d34-0410-b5e6-96231b3b80d8

chubbymaggie/asap,chubbymaggie/asap,dslab-epfl/asap,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,chubbymaggie/asap,apple/swift-llvm,GPUOpen-Drivers/llvm,dslab-epfl/asap,dslab-epfl/asap,chubbymaggie/asap,llvm-mirror/llvm,GPUOpen-Drivers/llvm,dslab-epfl/asap,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,dslab-epfl/asap,llvm-mirror/llvm,chubbymaggie/asap,dslab-epfl/asap,dslab-epfl/asap,chubbymaggie/asap,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm