[patch] Use alias linkage and visibility to decide tls access mode

[Rafael Espíndola]

This matches both what we do for the non-thread case and what gcc does.

I found this while looking a the possibility of having a GlobalAlias
with an aliasee that is an arbitrary ConstantExpr, and therefore with
no notion of "aliased global".

With this patch clang would match gcc's behaviour in

static __thread int a = 42;
extern __thread int b __attribute__((alias("a")));
int *f(void) { return &a; }
int *g(void) { return &b; }

if not for pr19843. Manually writing the IL does produce the same access modes.

It is also a step in the direction of fixing pr19844.

Cheers,
Rafael
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