[Target] Pass eSymbolContextEverything to GetSymbolContext().
[Target] Pass eSymbolContextEverything to GetSymbolContext().

Pointed out by Jason. While here, simplify the surrounding code.

<rdar://problem/46474594>

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb