[PECOFF] Handle .lib files as if they are grouped by --{start,end}-group.

Currently we do not de-duplicate library files specified by /defaultlib option.
As a result, the same files are added multiple times to the input graph. In
particular, some popular files, such as kernel32.lib or oldnames.lib, are added
more than 10 times during linking of LLD. That makes the linker slower, as it
needs to parse the same file again and again.

This patch solves the issue by de-duplicating. The same file will be added only
once to the input graph. This patch improved the LLD linking time from 10.5
seconds to 7.7 seconds on my 4-core Core i7 Macbook Pro.