[lldb/SymbolFile] Don't parse the whole line table for the support files

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.

With the old behavior, 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.

With the current behavior, 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.

Differential revision: https://reviews.llvm.org/D81589