Fix support for function multiple forward arguments - close #279 (#293)

* Add support for function multiple forward arguments

The current implementation correctly handle one forward argument
- it produces correct requires clause.

When there are more forward arguments like in following code:
```cpp
fun: (forward s1 : std::string, forward s2 : std::string, forward s3 : std::string) = {
    // ...
}
```
generates:
```cpp
requires std::is_same_v<CPP2_TYPEOF(s1), std::string>requires std::is_same_v<CPP2_TYPEOF(s2), std::string>requires std::is_same_v<CPP2_TYPEOF(s3), std::string>
```

This change correct this behaviour and creates requires clause
that joins multiple clauses with `&&`:
```cpp
requires (std::is_same_v<CPP2_TYPEOF(s1), std::string> && std::is_same_v<CPP2_TYPEOF(s2), std::string> && std::is_same_v<CPP2_TYPEOF(s3), std::string>)
```

* Add regression test for multiple forward arguments

Author: filipsajdak

regression-tests/pure2-function-multiple-forward-arguments.cpp2

@@ -0,0 +1,10 @@
+fun: (forward s1 : std::string, forward s2 : std::string, forward s3 : std::string) = {
+    std::cout << s1 << s2 << s3 << std::endl;
+}
+
+main: () = {
+    b : std::string = "b";
+    c : std::string = "c";
+    fun(std::string("a"), b, c);
+    b = "";
+}

regression-tests/test-results/apple-clang-14/pure2-function-multiple-forward-arguments.cpp.execution

@@ -0,0 +1 @@
+abc

regression-tests/test-results/apple-clang-14/pure2-function-multiple-forward-arguments.cpp.output

@@ -0,0 +1 @@
+abc

regression-tests/test-results/clang-12/pure2-function-multiple-forward-arguments.cpp.execution

@@ -0,0 +1,27 @@
+
+#define CPP2_USE_MODULES         Yes
+
+#include "cpp2util.h"
+
+
+#line 1 "pure2-function-multiple-forward-arguments.cpp2"
+auto fun(auto&& s1,             auto&&   s2,              auto&&   s3) -> void;
+#line 5 "pure2-function-multiple-forward-arguments.cpp2"
+auto main() -> int;
+
+//=== Cpp2 definitions ==========================================================
+
+#line 1 "pure2-function-multiple-forward-arguments.cpp2"
+auto fun(auto&& s1,             auto&&   s2,              auto&&   s3) -> void
+requires (std::is_same_v<CPP2_TYPEOF(s1), std::string> && std::is_same_v<CPP2_TYPEOF(s2), std::string> && std::is_same_v<CPP2_TYPEOF(s3), std::string>)
+#line 2 "pure2-function-multiple-forward-arguments.cpp2"
+{   std::cout << CPP2_FORWARD(s1) << CPP2_FORWARD(s2) << CPP2_FORWARD(s3) << std::endl;
+}
+
+auto main() -> int{
+    std::string b {"b"}; 
+    std::string c {"c"}; 
+    fun(std::string("a"), b, std::move(c));
+    b = "";
+}
+

regression-tests/test-results/clang-12/pure2-function-multiple-forward-arguments.cpp.output

@@ -0,0 +1,10 @@
+fun: (forward s1 : std::string, forward s2 : std::string, forward s3 : std::string) = {
+    std::cout << s1 << s2 << s3 << std::endl;
+}
+
+main: () = {
+    b : std::string = "b";
+    c : std::string = "c";
+    fun(std::string("a"), b, c);
+    b = "";
+}

regression-tests/test-results/pure2-function-multiple-forward-arguments.cpp

@@ -0,0 +1,2 @@
+pure2-function-multiple-forward-arguments.cpp2... ok (all Cpp2, passes safety checks)
+

regression-tests/test-results/pure2-function-multiple-forward-arguments.cpp2

@@ -4734,8 +4734,17 @@ class cppfront
             if (!function_requires_conditions.empty()) {
                 printer.ignore_alignment( true, n.position().colno + 4 );
                 printer.print_extra("\n");
-                for (auto const& req : function_requires_conditions) {
-                    printer.print_extra("requires " + req);
+                auto is_multi_fwd = function_requires_conditions.size() > 1;
+                printer.print_extra("requires ");
+                if (is_multi_fwd) {
+                    printer.print_extra("(");
+                }
+                printer.print_extra(function_requires_conditions.front());
+                for (auto it = std::cbegin(function_requires_conditions)+1; it != std::cend(function_requires_conditions); ++it) {
+                    printer.print_extra(" && " + *it);
+                }
+                if (is_multi_fwd) {
+                    printer.print_extra(")");
                 }
                 function_requires_conditions = {};
                 printer.ignore_alignment( false );