Partial revert of "Use std::foo_t rather than std::foo in LLVM." in googlebench

Since googlebench builds as c++11, the change there is incorrect and breaks the
googlebench build when the STL implementation is strict about std::enable_if_t
not being available in lesser c++ versions.

partial revert of: 1bd6123b (https://reviews.llvm.org/D74384)

Differential Revision: https://reviews.llvm.org/D101583