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/testdata/generic_function/nondeduced_generic_param.carbon

@@ -0,0 +1,22 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: result: 1
+
+package ExplorerTest api;
+class A(T:! Type) {
+  var v: T;
+}
+fn F(T:! Type, x: T) -> T {
+  var v: A(T) = {.v = x};
+  return v.v;
+}
+fn Main() -> i32 {
+  return F(i32, 1);
+}