[dmd-internals] Three critical regression patches

[Don Clugston]

The first one is a very nasty wrong-code regression.
The other two ICE bugs were introduced in the last two releases.

4524 Regression(2.026) Bus error with nested struct
4938 Regression(2.047) dmd segfault when compiling
4926 ICE: PREC_zero assertion failure due to unset precedence

and this one is important

3175 rejects templated ref return function

These other patches are less important, but straightforward:
4925 [ICE] segfault with module-scope assert(0)
4941 Built-in tuple slice boundaries are not CTFE'd
4866 Static-to-dynamic converted manifest constant array gets
non-converted type in static/constraint if