[lldb][CPlusPlus] Introduce CPlusPlusLanguage::MethodName::GetReturnType

This patch adds a way to extract the return type out
of the `CPlusPlusNameParser`. This will be useful
for cases where we want a function's basename *and* the
return type but not the function arguments; this is
currently not possible (the parser either gives us the
full name or just the basename). Since the parser knows
how to handle return types already we should just expose
this to users that need it.

**Testing**

* Added unit-tests

Differential Revision: https://reviews.llvm.org/D136935

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