[PECOFF][Driver] Add a library file even if it does not exist.

Missing files will be reported as errors in the later pass, so this patch
does not change the behavior of the LLD linker, but it helps writing unit
tests for the driver.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@187256 91177308-0d34-0410-b5e6-96231b3b80d8
[PECOFF][Driver] Add ".lib" extension to the path given with /defaultlib.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@187259 91177308-0d34-0410-b5e6-96231b3b80d8