Use std::foo_t rather than std::foo in LLVM.

Summary: C++14 migration. No functional change.

Reviewers: bkramer, JDevlieghere, lebedev.ri

Subscribers: MatzeB, hiraditya, jkorous, dexonsmith, arphaman, kadircet, lebedev.ri, usaxena95, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D74384