[OpenCL] Fix assertion due to blocks
A recent change caused assertion in CodeGenFunction::EmitBlockCallExpr when a block is called.
There is code
Func = CGM.getOpenCLRuntime().getInvokeFunction(E->getCallee());
getCalleeDecl calls Expr::getReferencedDeclOfCallee, which does not handle
BlockExpr and returns nullptr, which causes isa to assert.
This patch fixes that.
Differential Revision: https://reviews.llvm.org/D58658