Greg noticed that line table parsing has regressed by 5x in Xcode 11.4.1. Rephrased from our (off list) e-mail conversation:
Prior to my patch of using the LLVM line table parsing code, SymbolFileDWARF::ParseSupportFiles would only parse the line table prologues to get the file list for any files that could be in the line table. If we found the file that someone is setting the breakpoint in in the support files list, we would get a valid index. If we didn't, we would not look any further. So someone sets a breakpoint one "MyFile.cpp:12" and if we find "MyFile.cpp" in the support file list for the compile unit, then and only then would we get the entire line table for that compile unit. Now, no matter what, we always fully parse the line table for all compile units any time any file and line breakpoint is set. This creates a serious problem when debugging a large DWARF in .o file project.
This patch re-instates the old behavior. Unfortunately it means we might end up parsing to prologue twice, but I don't think that outweighs the cost of trying to cache/reuse it.