[lldb][nfc] Simplify DebugRanges class

Most of the code changed here dates back to 2010, when LLDB was first
introduced upstream, as such it benefits from a slight cleanup.

The method "dump" is not used anywhere nor is it tested, so this commit removes
it.

The "findRanges" method returns a boolean which is never checked and indicates
whether the method found anything/assigned a range map to the out parameter.
This commit folds the out parameter into the return type of the method.

A handful of typedefs were also never used and therefore removed.

Differential Revision: https://reviews.llvm.org/D150363