[CFG] Make representation of destructor calls more accurate.

Respect C++17 copy elision; previously it would generate destructor calls
for elided temporaries, including in initialization and return statements.

Don't generate duplicate destructor calls for statement expressions.

Fix destructors in initialization lists and comma operators.

Improve printing of implicit destructors.

Patch by Nicholas Allegra!

Differential Revision: https://reviews.llvm.org/D66404

llvm-svn: 370247