[libcxx] [test] Fix Clang -Wdeprecated-declarations with MSVC's CRT.

libcxx's tests use various C Standard Library functions that have been
marked by MSVC's CRT as deprecated by Microsoft (not by ISO).

libcxx's usage is cromulent (just checking with decltype to see if the functions
are being dragged in by various headers as required by the Standard), so
defining _CRT_SECURE_NO_WARNINGS will silence the warnings in a targeted manner.
This needs to be defined before including any CRT headers.

Also, make this file prettier.

Fixes D29138.

llvm-svn: 294157