[PECOFF] Add /alternatename option parser.

/ALTERNATENAME is a rarely-used, undocumented command line option that is
needed to link LLD for release build. It seems that the option is for defining
an weak alias; /alternatename:foo=bar defines weak symbol "foo" for "bar".
If "foo" is defined in an input file, it'll be linked normally and the command
line option will have no effect. If it's not defined, "foo" will be handled
as an alias for "bar".

This patch implements the parser for the option. The actual weak alias handling
will be implemented in a separate patch.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@196743 91177308-0d34-0410-b5e6-96231b3b80d8