[lldb] Fix gcc 5.4.0 compile error
Specify type when constructing PromotionKeys,
this fixes error:
"chosen constructor is explicit in copy-initialization"
when compiling lldb with GCC 5.4.0.
This is due to std::tuple having an explicit
default constructor, see:
Reviewed By: labath
Differential Revision: https://reviews.llvm.org/D86690