Fixing build bot error: adding const qualifiers to std::sort lambda.

Errors were not reproducible on clang-6.0 on ubuntu 16.04.

llvm-svn: 332192
diff --git a/llvm/lib/CodeGen/MIRCanonicalizerPass.cpp b/llvm/lib/CodeGen/MIRCanonicalizerPass.cpp
index a68ea19..a04e994 100644
--- a/llvm/lib/CodeGen/MIRCanonicalizerPass.cpp
+++ b/llvm/lib/CodeGen/MIRCanonicalizerPass.cpp
@@ -135,7 +135,7 @@
   }
 
   std::sort(StringInstrMap.begin(), StringInstrMap.end(),
-            [](StringInstrPair &a, StringInstrPair &b) {
+            [](const StringInstrPair &a, const StringInstrPair &b) -> bool {
               return (a.first < b.first);
             });