[LLDB]Fix buffer-over-flow bug introduced in 157170 (#159588)
If `pr_name` is longer than 16, it would be a non-null terminated
string. Assigning it to `std::string m_executable_name` would cause an
overflow read. Instead, just copy the name from thread_data.name.
To repro, run the `elf-core/TestLinuxCore.py` with asan
(Question: why is the new variable needed in the first place? can't the
thread_data.name be used?)