Extended Description

Consider this testcase:

---- A.cpp -----
struct X {
int A;
X();
};

extern const X y;

int foo(int *P) { int t = y.A; *P = 100; return t - y.A; }

Currently, llvmg++ marks 'y' as being llvm 'constant', allowing "foo" to be
optimized to just a single store. However, this is incorrect, because X has a
non-trivial ctor. The ctor could look like this:

---- B.cpp ----
struct X {
int A;
X();
};
int foo(int* P);

X::X() { printf("%d", foo(&A)); }

... in which the optimization breaks the program. This is a conformant C++
program, which we should not break.

The fix is conceptually straight-forward: globals should not be marked constant
if they have nontrivial ctor/dtors.

-Chris