[OpenACC] Fix crash on error recovery of variable in OpenACC mode

As reported, OpenACC's variable declaration handling was assuming some
semblence of legality in the example, so it didn't properly handle an
error case.  This patch fixes its assumptions so that we don't crash.

Fixes llvm#154008

Author: erichkeane

clang/lib/Sema/SemaOpenACC.cpp

@@ -1921,8 +1921,13 @@ void SemaOpenACC::ActOnVariableDeclarator(VarDecl *VD) {
     return;
 
   // This cast should be safe, since a static-local can only happen in a
-  // function declaration.
-  auto *ContextDecl = cast<FunctionDecl>(getCurContext());
+  // function declaration. However, in error cases (or perhaps ObjC/C++?), this
+  // could possibly be something like a 'block' decl, so if this is NOT a
+  // function decl, just give up.
+  auto *ContextDecl = dyn_cast<FunctionDecl>(getCurContext());
+
+  if (!ContextDecl)
+      return;
 
   // OpenACC 3.3 2.15:
   // In C and C++, function static variables are not supported in functions to

clang/test/SemaOpenACC/gh154008.cpp

@@ -0,0 +1,3 @@
+// RUN: %clang_cc1 %s -fopenacc -verify
+
+void *a = ^ { static int b };