Add a new flag and attributes to control static destructor registration

This commit adds the flag -fno-c++-static-destructors and the attributes
[[clang::no_destroy]] and [[clang::always_destroy]]. no_destroy specifies that a
specific static or thread duration variable shouldn't have it's destructor
registered, and is the default in -fno-c++-static-destructors mode.
always_destroy is the opposite, and is the default in -fc++-static-destructors
mode.

A variable whose destructor is disabled (either because of
-fno-c++-static-destructors or [[clang::no_destroy]]) doesn't count as a use of
the destructor, so we don't do any access checking or mark it referenced. We
also don't emit -Wexit-time-destructors for these variables.

rdar://21734598

Differential revision: https://reviews.llvm.org/D50994

llvm-svn: 340306
diff --git a/clang/test/CodeGenCXX/no_destroy.cpp b/clang/test/CodeGenCXX/no_destroy.cpp
new file mode 100644
index 0000000..93e6ce1
--- /dev/null
+++ b/clang/test/CodeGenCXX/no_destroy.cpp
@@ -0,0 +1,31 @@
+// RUN: %clang_cc1 %s -emit-llvm -triple x86_64-apple-macosx10.13.0 -o - | FileCheck %s
+
+struct NonTrivial {
+  ~NonTrivial();
+};
+
+// CHECK-NOT: __cxa_atexit{{.*}}_ZN10NonTrivialD1Ev
+[[clang::no_destroy]] NonTrivial nt1;
+// CHECK-NOT: _tlv_atexit{{.*}}_ZN10NonTrivialD1Ev
+[[clang::no_destroy]] thread_local NonTrivial nt2;
+
+struct NonTrivial2 {
+  ~NonTrivial2();
+};
+
+// CHECK: __cxa_atexit{{.*}}_ZN11NonTrivial2D1Ev
+NonTrivial2 nt21;
+// CHECK: _tlv_atexit{{.*}}_ZN11NonTrivial2D1Ev
+thread_local NonTrivial2 nt22;
+
+void f() {
+  // CHECK: __cxa_atexit{{.*}}_ZN11NonTrivial2D1Ev
+  static NonTrivial2 nt21;
+  // CHECK: _tlv_atexit{{.*}}_ZN11NonTrivial2D1Ev
+  thread_local NonTrivial2 nt22;
+}
+
+// CHECK: __cxa_atexit{{.*}}_ZN10NonTrivialD1Ev
+[[clang::always_destroy]] NonTrivial nt3;
+// CHECK: _tlv_atexit{{.*}}_ZN10NonTrivialD1Ev
+[[clang::always_destroy]] thread_local NonTrivial nt4;