Optimize AliasAssign tuple building

[BorisCarvajal]

Try to remove some quadratic behavior by expanding AliasSeq elements only in the target AliasDeclaration.

A synthetic test:

alias AliasSeq(T...) = T;
template Count(size_t l)
{
    alias Count = AliasSeq!();
    static foreach (i; 0 .. l)
        Count = AliasSeq!(Count, i);
}
alias c = Count!10000;
static assert(c.length == 10000);
static assert(c[0..5]   == AliasSeq!(0,1,2,3,4));
static assert(c[$-5..$] == AliasSeq!(9995,9996,9997,9998,9999));

Compilation time, memory usage. (just using /usr/bin/time -v dmd -o- test.d)
before: 6.66 sec. 815MB
after: 0.07 sec. 45MB

[dlang-bot]

Thanks for your pull request, @BorisCarvajal!

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Testing this PR locally

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dub run digger -- build "master + dmd#14332"