Add additional method for ObjectFileMachO to find the first loadable segment in a Mach-O binary

[jasonmolenda]

Summary

When lldb is processing the first couple of binaries in a process launch, it needs to compute load addresses before the files' sections have been added to the TargetSectionList, so our normal mechanism is not sufficient. ObjectFileMachO::ParseSymtab() calls GetMachHeaderSection() to find that segment, then uses the difference between that segment and the LINKEDIT segment, plus the Mach-O header load address that we were given, to calculate the load address of LINKEDIT.

This scheme works until one of the binaries we're treating in this special way is in the shared cache. GetMachHeaderSection() explicitly looks for a segment at file offset 0, as a way of implying the __TEXT segment without hardcoding its name. In a file in the shared cache, it will have a non-zero file address. It's laudable that ParseSymtab already handles the offset calculation correctly for a non-zero file offset -- but it won't match a segment which is nonzero.

This patch adds a backup mechanism if we find no segment at file offset 0, to find the __TEXT segment and return that.

Outside of an environment where one of these very initial binaries are in the shared cache, this can't be reproduced. Individual Mach-O binaries have file __TEXT with a file offset of 0. I haven't thought of a way to construct a well formed Mach-O binary that behaves like this, outside of the shared cache binary blob.

[jasonmolenda]

Fix typeo in comment.

[JDevlieghere]

LGTM

lldb/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp

Section *ObjectFileMachO::GetMachHeaderSection() {
   if (!section_list)
     return nullptr;
   const size_t num_sections = section_list->GetSize();
   for (size_t sect_idx = 0; sect_idx < num_sections; ++sect_idx) {
     Section *section = section_list->GetSectionAtIndex(sect_idx).get();
     if (section->GetFileOffset() == 0 && SectionIsLoadable(section))
       return section;
   }
+
+  // We may have a binary in the shared cache that has a non-zero
+  // file address for its first segment, traditionally the __TEXT segment.
+  // Search for it by name and return it as our next best guess.
+  SectionSP text_segment_sp =
+      GetSectionList()->FindSectionByName(GetSegmentNameTEXT());
+  if (text_segment_sp.get() && SectionIsLoadable(text_segment_sp.get()))
+    return text_segment_sp.get();
+
   return nullptr;
 }
 
 bool ObjectFileMachO::SectionIsLoadable(const Section *section) {
   if (!section)
     return false;
   const bool is_dsym = (m_header.filetype == MH_DSYM);
   if (section->GetFileSize() == 0 && !is_dsym)