C++ PATCH for c++/68206, 68530 (ICE with loop in constexpr)

[Jason Merrill]

Well-formed C++14 code with a loop in a constexpr function works fine, 
but we were crashing while trying to diagnose an unsuitable constexpr 
function because potential_constant_expression_1 didn't understand loops.

The second patch improves constexpr handling of EXIT_EXPR and loops 
around COMPOUND_EXPR rather than STATEMENT_LIST.  This is not currently 
necessary, but might be in future.

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