Allow .dSYM's to be directly placed in an alternate directory

Once available in the relevant toolchains this will allow us to implement
LLVM_EXTERNALIZE_DEBUGINFO_OUTPUT_DIR after D84127 by directly placing the dSYM
in the desired location instead of emitting next to the output file and moving
it.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D84572