[PECOFF] Fix parallel read bug.

InputElement::parse() may recursively call WinLinkDriver::parse() to handle
.drectve section contents, and if /defaultlib option exists in the section,
the driver will mutate the input graph to add a new input file to the graph.
So the access to input graph needs to be protected with mutext.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@204285 91177308-0d34-0410-b5e6-96231b3b80d8