swift234

fix <rdar://problem/19711233> QoI: poor diagnostic for operator-assignment involving immutable operand

We now produce tailored diagnostics for assignment operators that are passed a non-mutable LHS,
e.g.:

t.swift:14:3: error: cannot pass 'let' value 'x' to mutating binary operator '/='
x /= 19
~ ^
t.swift:13:1: note: change 'let' to 'var' to make it mutable
let x = 42
^~~
var





Swift SVN r27780