[lldb/Plugins] Replace platform-specific macro with LLVM_PRETTY_FUNCTION (NFC)

This patch refactors Scripted Process and Scripted Thread related
classes to use LLVM_PRETTY_FUNCTION instead of the compiler macro.

Differential Revision: https://reviews.llvm.org/D111452



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>