Don't assert if we can't constant fold extract/insertvalue

A non-constant-foldable static initializer expression containing insertvalue or
extractvalue had been causing an assert:

  Constants.cpp:1971: Assertion `FC && "ExtractValue constant expr couldn't be
                                 folded!"' failed.

Now we report a more-sensible "Unsupported expression in static initializer"
error instead.

Fixes PR15417.