Alternate fix to PR12248: put Sema in charge of special-casing

Alternate fix to PR12248:  put Sema in charge of special-casing
the diagnostic for assigning to a copied block capture.  This has
the pleasant side-effect of letting us special-case the diagnostic
for assigning to a copied lambda capture as well, without introducing
a new non-modifiable enumerator for it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@152593 91177308-0d34-0410-b5e6-96231b3b80d8