Fix DensetSet::insert_as() for MSVC2015 (NFC)

The latest MSVC update apparently resolve the call from the
const ref variant to itself, leading to an infinite
recursion. It is not clear to me why the r-value overload is
not selected. `ValueT` is a pointer type, and the functional-style
cast in the call `insert_as(ValueT(V), LookupKey);` should result
in a r-value ref. A bug in MSVC?

Differential Revision: https://reviews.llvm.org/D23956

llvm-svn: 280685