[BUG] Function initialized with == can't have _requires-clause_

[JohelEGP]

Title: Function initialized with == can't have requires-clause.

Description:

This is similar to commit b1c03f2 and #675.
Although the refactor of the latter would've made the fix trivial.
Now what commit b1c03f2 did for aliases
needs to be done for functions
after the gift of commit b589f5d.

Minimal reproducer (https://cpp2.godbolt.org/z/4exaWaf53):

f: () -> int requires true == 0;
main: () = { }

Commands:

cppfront main.cpp2
clang++18 -std=c++23 -stdlib=libc++ -lc++abi -pedantic-errors -Wall -Wextra -Wconversion -Werror=unused-result -I . main.cpp

Expected result: A well-formed program.

Actual result and error:

main.cpp2...
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<=>)

[JohelEGP]

A braced body does work (https://cpp2.godbolt.org/z/dadf8h4EM):

f: (_) -> int requires true == { return 0; }
main: () = { }