[lldb] Refactor host::OpenFileInExternalEditor

This patch refactors the macOS implementation of
OpenFileInExternalEditor. It fixes an AppleEvent memory leak, the
caching of LLDB_EXTERNAL_EDITOR and speculatively fixes a crash when
CFURL is NULL (rdar://108633464). The new code also improves error
handling, readability and documents calls to the CoreFoundation Launch
Services APIs.

A bunch of the Launch Services APIs have been deprecated
(LSFindApplicationForInfo, LSOpenURLsWithRole). The preferred API is
LSOpenCFURLRef but it doesn't specifying the "location" Apple Event
which is used to highlight the current line and switching over would
regress the existing behavior.

rdar://108633464

Differential revision: https://reviews.llvm.org/D149482