Emit const variables only once (#13614)

Otherwise they are emitted as internal and we get double-construction
and -destruction on the same memory address due to the way we promote
internal declarations in KeepLocalGVPass.

According to upstream tests, the de-duplication doesn't work on Windows
(yet), but I think this problem is severe enough to fix it on the other
platforms sooner rather than later.

Fixes #13429

test/CodeGeneration/const.C

@@ -0,0 +1,37 @@
+//------------------------------------------------------------------------------
+// CLING - the C++ LLVM-based InterpreterG :)
+//
+// This file is dual-licensed: you can choose to license it under the University
+// of Illinois Open Source License or the GNU Lesser General Public License. See
+// LICENSE.TXT for details.
+//------------------------------------------------------------------------------
+
+// RUN: cat %s |  %cling 2>&1 | FileCheck %s
+
+extern "C" int printf(const char*, ...);
+
+struct A {
+  int val;
+  A(int v) : val(v) {
+    printf("A(%d), this = %p\n", val, this);
+  }
+  ~A() {
+    printf("~A(%d), this = %p\n", val, this);
+  }
+  int getVal() const { return val; }
+};
+
+const A a(1);
+// CHECK: A(1), this = [[PTR:.+]]
+
+a.val
+// CHECK-NEXT: (const int) 1
+a.getVal()
+// CHECK-NEXT: (int) 1
+a.val
+// CHECK-NEXT: (const int) 1
+a.getVal()
+// CHECK-NEXT: (int) 1
+
+// CHECK-NEXT: ~A(1), this = [[PTR]]
+// CHECK-NOT: ~A