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

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