[CMake] Allow custom extensions for externalized debug info
Extra flexibility for emitting debug info to external files (remains Darwin only for now).
LLDB needs this functionality to emit a LLDB.framework.dSYM instead of LLDB.dSYM when building the framework, because the latter could conflict with the driver's lldb.dSYM when emitted in the same directory on case-insensitive file systems.
Reviewers: friss, bogner, beanz
Subscribers: mgorny, aprantl, llvm-commits, #lldb
Differential Revision: https://reviews.llvm.org/D60862