[OpenMP] Fix mapping of scalars for combined directives

Combined directives like 'target parallel' have two captured statements.
Sema has to check the right one from the right direction.

Previously, Sema::IsOpenMPCapturedByRef would return false for mapped
scalars on combined directives. This results in a wrong signature of
the outlined function which triggers an assertion:
void llvm::CallInst::init(llvm::FunctionType *, llvm::Value *, ArrayRef<llvm::Value *>, ArrayRef<OperandBundleDef>, const llvm::Twine &): Assertion `(i >= FTy->getNumParams() || FTy->getParamType(i) == Args[i]->getType()) && "Calling a function with a bad signature!"' failed.

Fixes PR30975 (and PR31985). New function was taken from clang-ykt.

Differential Revision: https://reviews.llvm.org/D34888

llvm-svn: 306956