[MC] Allow assembling .secidx and .secrel32 for undefined symbols

There's nothing incorrect about emitting such relocations against
symbols defined in other objects. The code in EmitCOFFSec* was missing
the visitUsedExpr part of MCStreamer::EmitValueImpl, so these symbols
were not being registered with the object file assembler.

This will be used to make reduced test cases for LLD.

llvm-svn: 306057