[PECOFF] Parse /merge command line option.

/merge:<from>=<to> option makes the linker to combine "from" section to "to"
section. This patch is to parse the option. The actual feature will be
implemented in a subsequent patch.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@193454 91177308-0d34-0410-b5e6-96231b3b80d8