libcxx: Rename .hpp files in libcxx/benchmarks to .h

LLVM uses .h as its extension for header files.

Differential Revision: https://reviews.llvm.org/D66509

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@369487 91177308-0d34-0410-b5e6-96231b3b80d8
13 files changed