Make wording for certain invalid unary expressions more consistent.

An invalid decltype expression like 'decltype int' gives:
error: expected '(' after 'decltype'

This makes it so 'sizeof int' gives a similar one:
error: expected parentheses around type name in sizeof expression


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192258 91177308-0d34-0410-b5e6-96231b3b80d8