Silence these C4715 warnings from Visual C++ (NFC)

llvm\tools\lld\lib\readerwriter\macho\macholinkingcontext.cpp(647):
warning C4715: 'lld::MachOLinkingContext::exportSymbolNamed' :
not all control paths return a value

llvm\tools\lld\lib\readerwriter\macho\machonormalizedfilefromatoms.cpp(723):
warning C4715: '`anonymous namespace'::Util::getSymbolTableRegion' :
not all control paths return a value

While all enum values do appear in the switch, an uninitialized or corrupted
enum variable would not be caught without the default: case in the switch.