[patch] Allow alias to point to an arbitrary ConstantExpr

[Rafael Espíndola]

Moving from llvm-dev to llvm-commits now that all the dependencies
have been committed and the direction seems agreed upon.

The attached patch changes GlobalAlias to point to an arbitrary
ConstantExpr and it is up to MC (or the system assembler) to decide if
that expression is valid or not.

This reduces our ability to diagnose invalid uses and how early we can
spot them, but it also lets us do things like

@test5 = alias inttoptr(i32 sub (i32 ptrtoint (i32* @test2 to i32),
                                 i32 ptrtoint (i32* @bar to i32)) to i32*)


An important implication of this patch is that the notion of aliased
global doesn't exist any more. The alias has to encode the information
needed to access it in its metadata (linkage, visibility, type, etc).

The clang patch includes a walk of alias chains, but that should be OK
since clang knows the form of every alias it creates.

The llvm patch also includes one as a local hack in the old JIT. That
is hopefully OK since we are about to delete it and it should handle
as many cases as before.

Another consequence to notice is that getSection has to return a
"const char *". It could return a NullTerminatedStringRef if there was
such a thing, but when that was proposed the decision was to just uses
"const char*" for that.

Cheers,
Rafael
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