[libclang] Avoid having the cursor of an expression replace the declaration cursor

[libclang] Avoid having the cursor of an expression replace the declaration cursor
when the expression source range overlaps the declaration range.

This can happen for C++ constructor expressions whose range generally
include the variable declaration, e.g.:

  MyCXXClass foo; // Make sure pointing at 'foo' returns a VarDecl cursor.

rdar://9124499.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@133930 91177308-0d34-0410-b5e6-96231b3b80d8