[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
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 15a297212fd42fddfca93b58dacbde1705f12d53
diff --git a/include/atomic b/include/atomic
index dc117e9..a0245eb 100644
--- a/include/atomic
+++ b/include/atomic
@@ -1689,7 +1689,7 @@
 #endif // _LIBCPP_HAS_NO_DEFAULTED_FUNCTIONS
 
     _LIBCPP_INLINE_VISIBILITY
-    atomic_flag(bool __b) _NOEXCEPT : __a_(__b) {}
+    atomic_flag(bool __b) _NOEXCEPT : __a_(__b) {} // EXTENSION
 
 #ifndef _LIBCPP_HAS_NO_DELETED_FUNCTIONS
     atomic_flag(const atomic_flag&) = delete;