[CMake] Add LLVM_EXTERNALIZE_DEBUGINFO_OUTPUT_DIR for custom dSYM target directory on Darwin

Summary: When using `LLVM_EXTERNALIZE_DEBUGINFO` in LLDB, the default dSYM location for the shared library in LLDB.framework is inside the framework bundle. With `LLVM_EXTERNALIZE_DEBUGINFO_OUTPUT_DIR` we can easily fix that. I consider it a useful feature to be able to set a global output directory for external debug info (rather then having a target-specific one). Only implemented for Darwin so far.

Reviewers: beanz, aprantl

Reviewed By: aprantl

Subscribers: mgorny, aprantl, #lldb, lldb-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D55114

llvm-svn: 348118