Revert "Revert 220932.": "Removing the static initializer in ManagedStatic.cpp by using llvm_call_once to initialize the ManagedStatic mutex"

This reverts commit r221331 and reinstate r220932 as discussed in D19271.
Original commit message was:

This patch adds an llvm_call_once which is a wrapper around
std::call_once on platforms where it is available and devoid
of bugs. The patch also migrates the ManagedStatic mutex to
be allocated using llvm_call_once.

These changes are philosophically equivalent to the changes
added in r219638, which were reverted due to a hang on Win32
which was the result of a bug in the Windows implementation
of std::call_once.

Differential Revision: http://reviews.llvm.org/D5922

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 269577

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/lib/Support/Unix/Threading.inc b/llvm/lib/Support/Unix/Threading.inc
new file mode 100644
index 00000000000..35ec876904a
--- /dev/null
+++ b/llvm/lib/Support/Unix/Threading.inc
@@ -0,0 +1,5 @@
+#include <thread>
+
+void llvm::call_once(once_flag& flag, void (*fptr)(void)) {
+  std::call_once(flag, fptr);
+}