[CMake] Allow custom extensions for externalized debug info

Summary:
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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60862

llvm-svn: 358685