[OpenMP] Fix accidental reuse of VLA size

We were using an OpaqueValueExpr allocated on the stack to store
the size of a VLA. Because the VLASizeMap in CodegenFunction
uses the address of the expression to avoid recomputing VLAs,
we were accidentally reusing an earlier llvm::Value. This led to
invalid LLVM IR.

This is a temporary solution until VLASizeMap can be pushed and popped
based on the context.

Differential Revision: https://reviews.llvm.org/D107666