Use llvm::StringSwitch.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@219341 91177308-0d34-0410-b5e6-96231b3b80d8
[PECOFF] Remember DLL names given with /delayload option.

This is a step toward full support of /delayload.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@219344 91177308-0d34-0410-b5e6-96231b3b80d8