[PECOFF] Handle hint field in Hint/Name table in the import library.

A hint is an index of the export pointer table in a DLL, at which
PE/COFF loader starts looking for a symbol name. The import library
comes with hints and symbol pairs, and as long as hints are in sync
with the actual symbol table in DLL, the symbols will be resolved
quickly. So, we shouldn't ignore hints but propagate them to an output.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@185516 91177308-0d34-0410-b5e6-96231b3b80d8