Improve the error message for assigning to read-only variables.

Improve the error message for assigning to read-only variables.

Previously, many error messages would simply be "read-only variable is not
assignable"  This change provides more information about why the variable is
not assignable, as well as note to where the const is located.

Differential Revision: http://reviews.llvm.org/D4479

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@234677 91177308-0d34-0410-b5e6-96231b3b80d8