[TargetMachine] Move COFF special case for ExternalSymbolSDNode from...

[TargetMachine] Move COFF special case for ExternalSymbolSDNode from shouldAssumeDSOLocal to X86Subtarget

Intended to be NFC. ARM/AArch64 don't appear to need adjustment.

TargetMachine::shouldAssumeDSOLocal is expected to be very simple, ideally
matching isDSOLocal(). The IR producers are expected to set dso_local correctly.
(While some may think this function can make producers' work easier, the
function is really not in a good position to set dso_local. See the various
special cases we duplicate from clang CodeGenModule.cpp.)

Reviewed By: mstorsjo

Differential Revision: https://reviews.llvm.org/D108514