[lldb] Use if-with-initializer pattern in SBTarget (NFC) (#141284)

Use the if statement with an initializer pattern that's very common in
LLVM in SBTarget. Every time someone adds a new method to SBTarget, I
want to encourage using this pattern, but I don't because it would be
inconsistent with the rest of the file. This solves that problem by
switching over the whole file.

Author: JDevlieghere