SymbolRewriter: use iplist::splice
SymbolRewriter: use iplist::splice

The swap implementation for iplist is currently unsupported.  Simply splice the
old list into place, which achieves the same purpose.  This is needed in order
to thread the -frewrite-map-file frontend option correctly.  NFC.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@225186 91177308-0d34-0410-b5e6-96231b3b80d8

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