[lldb] [ObjectFileMachO] LLVM_COV is not mapped into firmware memory (llvm#86359)

It is possible to gather code coverage in a firmware environment, where
the __LLVM_COV segment will not be mapped in memory but does exist in
the binary, see

https://llvm.org/devmtg/2020-09/slides/PhippsAlan_EmbeddedCodeCoverage_LLVM_Conf_Talk_final.pdf

The __LLVM_COV segment in the binary happens to be at the same address
as the __DATA segment, so if lldb treats this segment as loaded, it
shadows the __DATA segment and address->symbol resolution can fail.

For these non-userland code cases, we need to mark __LLVM_COV as not a
loadable segment.

rdar://124475661

Author: jasonmolenda

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

@@ -905,6 +905,11 @@ ConstString ObjectFileMachO::GetSegmentNameDWARF() {
   return g_section_name;
 }
 
+ConstString ObjectFileMachO::GetSegmentNameLLVM_COV() {
+  static ConstString g_section_name("__LLVM_COV");
+  return g_section_name;
+}
+
 ConstString ObjectFileMachO::GetSectionNameEHFrame() {
   static ConstString g_section_name_eh_frame("__eh_frame");
   return g_section_name_eh_frame;
@@ -6145,6 +6150,13 @@ bool ObjectFileMachO::SectionIsLoadable(const Section *section) {
     return false;
   if (GetModule().get() != section->GetModule().get())
     return false;
+  // firmware style binaries with llvm gcov segment do
+  // not have that segment mapped into memory.
+  if (section->GetName() == GetSegmentNameLLVM_COV()) {
+    const Strata strata = GetStrata();
+    if (strata == eStrataKernel || strata == eStrataRawImage)
+      return false;
+  }
   // Be careful with __LINKEDIT and __DWARF segments
   if (section->GetName() == GetSegmentNameLINKEDIT() ||
       section->GetName() == GetSegmentNameDWARF()) {

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

@@ -271,6 +271,7 @@ class ObjectFileMachO : public lldb_private::ObjectFile {
   static lldb_private::ConstString GetSegmentNameOBJC();
   static lldb_private::ConstString GetSegmentNameLINKEDIT();
   static lldb_private::ConstString GetSegmentNameDWARF();
+  static lldb_private::ConstString GetSegmentNameLLVM_COV();
   static lldb_private::ConstString GetSectionNameEHFrame();
 
   llvm::MachO::dysymtab_command m_dysymtab;