[ELF] Pass SectionPiece by reference in getSectionPiece. NFC (#190110)

The generated assembly looks more optimized. In addition, this avoids
widened load, which would cause a TSan-detected data race with parallel
--gc-sections (#189321).

lld/ELF/InputSection.cpp

@@ -1557,7 +1557,8 @@ SectionPiece &MergeInputSection::getSectionPiece(uint64_t offset) {
   if (!(flags & SHF_STRINGS))
     return pieces[offset / entsize];
   return partition_point(
-      pieces, [=](SectionPiece p) { return p.inputOff <= offset; })[-1];
+      pieces,
+      [=](const SectionPiece &p) { return p.inputOff <= offset; })[-1];
 }
 
 // Return the offset in an output section for a given input offset.