[PECOFF] Process Import Name/Type field in the import library.

This patch removes hacky mangle() function, which strips all decorations
uncondtitionally. LLD now interprets Import Name/Type field in the import
library properly as described in the Microsoft PE/COFF Spec.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@187388 91177308-0d34-0410-b5e6-96231b3b80d8