[analyzer] pr37688: Fix a crash upon evaluating a deleted destructor of a union.

Add a defensive check against an invalid destructor in the CFG.

Unions with fields with destructors have their own destructor implicitly
deleted. Due to a bug in the CFG we're still trying to evaluate them
at the end of the object's lifetime and crash because we are unable
to find the destructor's declaration.

rdar://problem/47362608

Differential Revision: https://reviews.llvm.org/D56899

llvm-svn: 351610