[BUG] Can't mix requires clause with simple function body #423

Title: Can't mix requires clause with simple function body.

Minimal reproducer (https://cpp2.godbolt.org/z/o6fcxPP9r):

f: () requires a = g();
main: () = { }

Commands:

cppfront -clean-cpp1 main.cpp2

Expected result:

a to be the requires's expression,
= g() to be f's initializer.

Actual result and error:

main.cpp2(1,1): error: a function must have a body ('=' initializer), unless it is virtual (has a 'virtual this' parameter) or is defaultable (operator== or operator<=>)

Suggested resolution:

 //G requires-clause:
-//G     'requires' expression
+//G     'requires' logical-or-expression