[libclang] evalute compound statement cursors before trying to evaluate

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@336590 91177308-0d34-0410-b5e6-96231b3b80d8