Work around bug in Clang 3.8

...that causes

> clang/lib/AST/Type.cpp:1878: bool clang::Type::isConstantSizeType() const: Assertion `!isDependentType() && "This doesn't make sense for dependent types"' failed.

compilation failure in PCodeBufferWalker in basic/source/comp/codegen.cxx

>    static T readParam( sal_uInt8 const *& pCode )
>    {
>        short nBytes = sizeof( T );
>        T nOp1=0;
>        for ( int i=0; i<nBytes; ++i )
>            nOp1 |= *pCode++ << ( i * 8);
>        return nOp1;
>    }

with loplugin:oncevar.

Change-Id: I25417076549ea538adf013282f3657e0d642d776

compilerplugins/clang/oncevar.cxx

@@ -162,11 +162,27 @@ bool OnceVar::VisitVarDecl( const VarDecl* varDecl )
             }
         }
     }
-    if (!foundStringLiteral
-        && !varDecl->getInit()->isConstantInitializer(compiler.getASTContext(), false/*ForRef*/))
+    if (!foundStringLiteral) {
+        auto const init = varDecl->getInit();
+#if CLANG_VERSION < 30900
+        // Work around missing Clang 3.9 fix <https://reviews.llvm.org/rL271762>
+        // "Sema: do not attempt to sizeof a dependent type" (while an
+        // initializer expression of the form
+        //
+        //   sizeof (T)
+        //
+        // with dependent type T /is/ constant, keep consistent here with the
+        // (arguably broken) behavior of isConstantInitalizer returning false in
+        // Clang >= 3.9):
+        if (init->isValueDependent()) {
+            return true;
+        }
+#endif
+        if (!init->isConstantInitializer(compiler.getASTContext(), false/*ForRef*/))
         {
             return true;
         }
+    }
 
     maVarDeclSet.insert(varDecl);