[libunwind] [libunwind] Faster handling of frames with missed FDE records. (PR #167849)

[Fangrui Song via cfe-commits]

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@@ -1787,20 +1787,37 @@ bool UnwindCursor<A, R>::getInfoFromDwarfSection(
   }
   if (!foundFDE) {
     // Still not found, do full scan of __eh_frame section.
-    foundFDE = CFI_Parser<A>::findFDE(_addressSpace, pc, sects.dwarf_section,
-                                      sects.dwarf_section_length, 0,
-                                      &fdeInfo, &cieInfo);
+    // But only if __eh_frame_hdr is absent or empty.
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MaskRay wrote:

`.eh_frame_hdr` (ELF) instead of `__eh_frame_hdr`. The latter uses Mach-O's naming convention but Mach-O doesn't use `eh_frame_hdr`.

https://github.com/llvm/llvm-project/pull/167849