Revert r372777: [libc++] Implement LWG 2510 and its follow-ups

This also reverts:
 - r372778: [libc++] Implement LWG 3158
 - r372782: [libc++] Try fixing tests that fail on GCC 5 and older
 - r372787: Purge mentions of GCC 4 from the test suite

Reason: the change breaks compilation of LLVM with libc++, for details see
http://lists.llvm.org/pipermail/libcxx-dev/2019-September/000599.html

llvm-svn: 372832
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: a3d337a9a7d00d82bb190c5e551181d3998f6b98
diff --git a/src/mutex.cpp b/src/mutex.cpp
index 7e979cd..1827857 100644
--- a/src/mutex.cpp
+++ b/src/mutex.cpp
@@ -21,9 +21,9 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 #ifndef _LIBCPP_HAS_NO_THREADS
 
-const defer_lock_t  defer_lock{};
-const try_to_lock_t try_to_lock{};
-const adopt_lock_t  adopt_lock{};
+const defer_lock_t  defer_lock = {};
+const try_to_lock_t try_to_lock = {};
+const adopt_lock_t  adopt_lock = {};
 
 // ~mutex is defined elsewhere
 
diff --git a/src/utility.cpp b/src/utility.cpp
index 6a690dc..016a5d9 100644
--- a/src/utility.cpp
+++ b/src/utility.cpp
@@ -10,6 +10,6 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-const piecewise_construct_t piecewise_construct{};
+const piecewise_construct_t piecewise_construct = {};
 
 _LIBCPP_END_NAMESPACE_STD