Fix gcc-05.4 bot failures caused by in r363481 "[clangd] Index API and implementations for relations"

Use std::make_tuple instead of initializer list to make gcc-5.4 happy.

See https://reviews.llvm.org/D62839 for details.

llvm-svn: 363504
diff --git a/clang-tools-extra/clangd/index/FileIndex.cpp b/clang-tools-extra/clangd/index/FileIndex.cpp
index a90c51b..e85fef5 100644
--- a/clang-tools-extra/clangd/index/FileIndex.cpp
+++ b/clang-tools-extra/clangd/index/FileIndex.cpp
@@ -72,7 +72,8 @@
        "  relations slab: {7} relations, {8} bytes",
        FileName, IsIndexMainAST, Syms.size(), Syms.bytes(), Refs.size(),
        Refs.numRefs(), Refs.bytes(), Relations.size(), Relations.bytes());
-  return {std::move(Syms), std::move(Refs), std::move(Relations)};
+  return std::make_tuple(std::move(Syms), std::move(Refs),
+                         std::move(Relations));
 }
 
 SlabTuple indexMainDecls(ParsedAST &AST) {