allow COFF .def directive in module assembly when using ThinLTO

Summary:
Using COFF's .def directive in module assembly used to crash ThinLTO
with "this directive only supported on COFF targets" when getting
symbol information in ModuleSymbolTable.  This change allows
ModuleSymbolTable to process such code and adds a test to verify that
the .def directive has the desired effect on the native object file,
with and without ThinLTO.

Fixes https://bugs.llvm.org/show_bug.cgi?id=36789

Reviewers: rnk, pcc, vlad.tsyrklevich

Subscribers: mehdi_amini, eraman, hiraditya, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D57073

llvm-svn: 352112