Make FullExprArg trivially copyable by removing the copy ctor

The default copy ctor is good.  This avoids using inalloca for a bunch
of Sema methods when self-hosting.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215047 91177308-0d34-0410-b5e6-96231b3b80d8