Avoid mutating variables that require const initialization

We cannot mutate variable declarations that require constant
initialization as Dredd's mutations occur dynamically.

Fixes: #235.

src/libdredd/src/mutate_visitor.cc

@@ -118,7 +118,7 @@ bool MutateVisitor::TraverseDecl(clang::Decl* decl) {
     return true;
   }
   if (const auto* var_decl = llvm::dyn_cast<clang::VarDecl>(decl)) {
-    if (var_decl->isConstexpr()) {
+    if (var_decl->isConstexpr() || var_decl->hasConstantInitialization()) {
       // Because Dredd's mutations occur dynamically, they cannot be applied to
       // C++ constexprs, which require compile-time evaluation.
       return true;

test/single_file/const_init.cc

@@ -0,0 +1,3 @@
+int main() {
+  [[clang::require_constant_initialization]] static int x = 0;
+}

test/single_file/const_init.cc.expected

@@ -0,0 +1,3 @@
+int main() {
+  [[clang::require_constant_initialization]] static int x = 0;
+}

test/single_file/const_init.cc.noopt.expected

@@ -0,0 +1,3 @@
+int main() {
+  [[clang::require_constant_initialization]] static int x = 0;
+}