[FormatEntity] Add mangled function name support

Summary:
Add `function.mangled-name` key for FormatEntity to show the mangled
function names in backtraces.

rdar://54088244



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

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D71237



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