[COFF] Ignore semicolons in module definition identifiers

Patch by David Major.

The NSS project's .def files make heavy use of semicolons in a
frightening attempt at portability:
https://hg.mozilla.org/projects/nss/raw-file/tip/lib/ckfw/capi/nsscapi.def

lld-link was treating the semicolon as part of the export name,
resulting in unresolved symbols. This patch includes ';' in the list of
characters to split on.

Differential Revision: https://reviews.llvm.org/D39968

llvm-svn: 319933