C++ PATCH for c++/69261 (ICE with constexpr and arrays)

[Jason Merrill]

In my fix for 67104 I knew that I wasn't handling RANGE_EXPRs, but 
thought that it wasn't necessary, and left an assert to make sure. 
Apparently I was wrong, so this patch implements the necessary range 
splitting when the constexpr code wants to store to a single element of 
a range.

Tested x86_64-pc-linux-gnu, applying to trunk.
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