Check that Symbol types are trivially destructible

This adds an extra level of static safety to our use of placement
new to allocate Symbol types.  It prevents the accidental addition
on a non-trivially-destructible member that could allocate and
leak memory.

From the spec: Storage occupied by trivially destructible objects
may be reused without calling the destructor.

Differential Revision: https://reviews.llvm.org/D43244

llvm-svn: 325025