Rename StringMap::emplace_second to try_emplace.

Coincidentally this function maps to the C++17 try_emplace. Rename it
for consistentcy with C++17 std::map. NFC.

llvm-svn: 276276
diff --git a/llvm/lib/Transforms/Instrumentation/GCOVProfiling.cpp b/llvm/lib/Transforms/Instrumentation/GCOVProfiling.cpp
index 32beab7..37f3cee 100644
--- a/llvm/lib/Transforms/Instrumentation/GCOVProfiling.cpp
+++ b/llvm/lib/Transforms/Instrumentation/GCOVProfiling.cpp
@@ -251,7 +251,7 @@
   class GCOVBlock : public GCOVRecord {
    public:
     GCOVLines &getFile(StringRef Filename) {
-      return LinesByFile.emplace_second(Filename, Filename, os).first->second;
+      return LinesByFile.try_emplace(Filename, Filename, os).first->second;
     }
 
     void addEdge(GCOVBlock &Successor) {