PR11410: Extend diagnostic to cover all cases of aggregate initialization, not
just the extremely specific case of a trailing array element that couldn't be
initialized because the default constructor for the element type is deleted.

Also reword the diagnostic to better match our other context diagnostics and to
prepare for the implementation of core issue 1070.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210083 91177308-0d34-0410-b5e6-96231b3b80d8