ClangFunction: Fix destruction order of parser and execution unit

Fix PR21802 by correcting the destruction order of
`ClangExpressionParser` and `IRExecutionUnit` in `ClangFunction`.  The
former has hooks into the latter -- i.e., `clang::CGDebugInfo` points at
the `LLVMContext` -- so it needs to be torn down first.

This was exposed by r223802 in LLVM, which started doing work in the
`CGDebugInfo` teardown.