[modules] Stop trying to fake up a linear MacroDirective history.

[modules] Stop trying to fake up a linear MacroDirective history.

Modules builds fundamentally have a non-linear macro history. In the interest
of better source fidelity, represent the macro definition information
faithfully: we have a linear macro directive history within each module, and at
any point we have a unique "latest" local macro directive and a collection of
visible imported directives. This also removes the attendent complexity of
attempting to create a correct MacroDirective history (which we got wrong
in the general case).

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@236176 91177308-0d34-0410-b5e6-96231b3b80d8