[CMake] Add option LLVM_EXTERNALIZE_DEBUGINFO

Summary: This adds support for generating dSYM files and stripping debug info from executables and dylibs. It also supports passing -object_path_lto to the linker to generate dSYMs for LTO builds.

Reviewers: bogner, friss

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D15133

llvm-svn: 254627