[PECOFF] Resolve __delayLoadHelper2 if /delayload is given

DLL delay importing is a feature to load a DLL lazily, instead of
at program start-up time.

If the feature is turned on with the /delayload flag, the linker
resolves the delay-load helper function. All function pointer table
entries for the DLL are initially pointing to the helper function.
When called, the function loads and resolves the DLL symbols using
dlopen-ish Windows system calls and then write the reuslts to the
function pointer table. The helper function is in "delayimp.lib".

Note that this feature is not completely implemented yet. LLD
also needs to emit the table that's consumed by the delay-load
helper function. That'll be done in another patch.

llvm-svn: 218943