Remove use of is_trivially_constructible.

type_traits header in libstdc++ 4.8 does not define is_trivially_contructible
so the code doesn't compile with it.

In this file we are using the trait for assertion to provide a better
error message. Removing it doesn't change the meaning of the code.

Differential Revision: http://reviews.llvm.org/D20719

llvm-svn: 270957