[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:
http://cplusplus.github.io/LWG/lwg-defects.html#2193

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D86690