[MCDwarf] Use emplace to avoid move assignment. NFC

(cherry picked from commit b0c4cd3)

Author: MaskRay

llvm/lib/MC/MCDwarf.cpp

@@ -266,7 +266,7 @@ void MCDwarfLineTable::emit(MCStreamer *MCOS, MCDwarfLineTableParams Params) {
   // In a v5 non-split line table, put the strings in a separate section.
   Optional<MCDwarfLineStr> LineStr;
   if (context.getDwarfVersion() >= 5)
-    LineStr = MCDwarfLineStr(context);
+    LineStr.emplace(context);
 
   // Switch to the section where the table will be emitted into.
   MCOS->switchSection(context.getObjectFileInfo()->getDwarfLineSection());

llvm/unittests/MC/DwarfLineTableHeaders.cpp

@@ -120,7 +120,7 @@ class DwarfLineTableHeaders : public ::testing::Test {
     MCDwarfLineTableParams Params = Assembler.getDWARFLinetableParams();
     Optional<MCDwarfLineStr> LineStr(None);
     if (Ctx.getDwarfVersion() >= 5) {
-      LineStr = MCDwarfLineStr(Ctx);
+      LineStr.emplace(Ctx);
       Header.setRootFile("dir", "file", None, None);
     }
     MCSymbol *LineEndSym = Header.Emit(TheStreamer, Params, LineStr).second;