Fix wrong-code bug when a const automatic variable of struct type has both a

mutable member and a constant initializer. We'd previously promoted such
variables to global constants, resulting in nasal demons if the mutable member
was modified.

This is only a temporary fix. The subtle interplay between isConstantInitializer
and CGExprConstant is very bug-prone; there are some other issues in this area
which I will be addressing in subsequent, more major reworking of this code.

llvm-svn: 145654