[lld][LinkingContext] Fix doxygen comment

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@190629 91177308-0d34-0410-b5e6-96231b3b80d8
[PECOFF] Handle weak external symbols.

Used the fallback mechanism to implement COFF weak external symbols.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@190633 91177308-0d34-0410-b5e6-96231b3b80d8