[libcxx] [test] Replace non-Standard "atomic_flag f(false);" with Standard "atomic_flag f;"

Summary:
Replace non-Standard "atomic_flag f(false);" with Standard "atomic_flag f;" in clear tests.
Although the  value of 'f' is unspecified it shouldn't matter because these tests always call `f.test_and_set()` without checking the result, so the initial state shouldn't matter.

The test init03.pass.cpp is explicitly testing this non-Standard extension; It has been moved into the `test/libcxx` directory.

Reviewers: mclow.lists, STL_MSFT

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D19758

llvm-svn: 268355