Upgrade frontend & libs to v2.091.0

[kinke]

No description provided.

[kinke]

Something bad must have happened wrt. static if and semantic analysis order or so:

core/stdc/math.d(4573): Error: undefined identifier `c_long`

when compiling core.stdc.config separately for non-Windows targets, which defines the c_long alias itself. It works when replacing the Posix-specific static if (https://github.com/dlang/druntime/blob/f80a07e1a0140f01878a4b13e2eea61faf6a3063/src/core/stdc/config.d#L118) by version (D_LP64).

[kinke]

Found a workaround for the c_long issue (localized import). The issue isn't new, e.g., LDC v1.20 also fails to compile core.stdc.config for Posix when importing core.math in core.internal.convert. The new global import was also a problem for neither-Windows-nor-Posix targets (wasm, Android...) because core.stdc.config.c_long isn't defined for those (but required in core.stdc.math...). It's LDC-specific because some core.math intrinsics are actually aliases to core.stdc.math symbols...

[kinke]

The only remaining problem is the ltsmaster compilability regression due to modernized contract syntax.

[kinke]

We're now at a stage where ltsmaster apparently miscompiles the compiler, so that new ownership-borrow tests cause LDC segfaults if compiled with ltsmaster (Travis job): compilable/ob1.d, fail_compilation/fob{1,2}.d.