Run clang-tidy's performance-unnecessary-copy-initialization over LLVM.

No functionality change intended.

llvm-svn: 272516
diff --git a/llvm/tools/llvm-mc/llvm-mc.cpp b/llvm/tools/llvm-mc/llvm-mc.cpp
index aaf8807..d75e24f 100644
--- a/llvm/tools/llvm-mc/llvm-mc.cpp
+++ b/llvm/tools/llvm-mc/llvm-mc.cpp
@@ -250,7 +250,7 @@
 
   bool Error = false;
   while (Lexer.Lex().isNot(AsmToken::Eof)) {
-    AsmToken Tok = Lexer.getTok();
+    const AsmToken &Tok = Lexer.getTok();
 
     switch (Tok.getKind()) {
     default: