[libclang] Recommit r336590 with a fix for the memory leak in the test

The original commit had a memory leak in the test has a leak as it doesn't
dispose of the evaluated cursor result.

This also contains the follow-up NFC refactoring commit r336591.

rdar://45893054

Original commit message:

[libclang] evalute compound statement cursors before trying to evaluate
the cursor like a declaration

This change fixes a bug in libclang in which it tries to evaluate a statement
cursor as a declaration cursor, because that statement still has a pointer to
the declaration parent.

rdar://38888477

Differential Revision: https://reviews.llvm.org/D49051

llvm-svn: 350666