Fix parsing of complicated C++ names

Summary:
CPlusPlusLanguage::MethodName was not correctly parsing templated functions whose demangled name
included the return type -- the space before the function name was included in the "context" and
the context itself was not terminated correctly due to a misuse of the substr function (second
argument is length, not the end position). Fix that and add a regression test.

Reviewers: clayborg

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D23608

llvm-svn: 279038