[ADT] Use explicit constructors for DenseMapPair to work around compiler issues.

Inheriting constructors from std::pair caused clang-3.8 to treat some DenseMap
initializer_list constructor calls as ambiguous, which broke several bots. This
commit explicitly defines DenseMapPair's constructos to work around the issue.

https://reviews.llvm.org/D53726

llvm-svn: 345411