[Object][COFF] Set the generic SF_Exported flag on COFF exported symbols.

The ORC ObjectLinkingLayer uses this flag during symbol lookup. Failure to set
it causes all symbols to behave as if they were non-exported, which has caused
failures in the kaleidoscope tutorials on Windows. Raising the flag should
un-break the tutorials.

No test case yet - none of the existing command line tools for printing symbol
tables (llvm-nm, llvm-objdump) show the status of this flag, and I don't want to
change the format from these tools without consulting their owners. I'll send an
email to the dev-list to figure out the right way forward.

llvm-svn: 258665