[llvm-profgen] Support symbol loading for debug fission

Support to load debug info from dwarf split file, like .dwo, .dwp files. Leverage the `getNonSkeletonUnitDIE(false)` API to achieve this.

Add test cause to make sure all the ranges is well retrieved by the loader.

Reviewed By: ayermolo, hoy, wenlei

Differential Revision: https://reviews.llvm.org/D115973