Initial support for generic parameters that are introduced in a function parameter list (#1247)

In order for this to work, we need to always use the argument deduction code path for function calls, instead of only using it when there is a `[...]` list, which means that argument deduction now needs to support implicit conversion.

Co-authored-by: Jon Meow <jperkins@google.com>

explorer/ast/pattern.h

@@ -17,6 +17,7 @@
 #include "explorer/ast/value_category.h"
 #include "explorer/common/source_location.h"
 #include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/STLFunctionalExtras.h"
 
 namespace Carbon {
 
@@ -88,11 +89,12 @@ class Pattern : public AstNode {
   std::optional<Nonnull<const Value*>> value_;
 };
 
-class BindingPattern;
-
-// Returns all `BindingPattern`s in the AST subtree rooted at `pattern`.
-auto GetBindings(const Pattern& pattern)
-    -> std::vector<Nonnull<const BindingPattern*>>;
+// Call the given `visitor` on all patterns nested within the given pattern,
+// including `pattern` itself. Aborts and returns `false` if `visitor` returns
+// `false`, otherwise returns `true`.
+auto VisitNestedPatterns(const Pattern& pattern,
+                         llvm::function_ref<bool(const Pattern&)> visitor)
+    -> bool;
 
 // A pattern consisting of the `auto` keyword.
 class AutoPattern : public Pattern {