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/implicit_conversion.carbon

@@ -0,0 +1,27 @@
+// 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: 4213
+
+package ExplorerTest api;
+
+interface IntLike {
+  fn Convert[me: Self]() -> i32;
+}
+impl i32 as IntLike {
+  fn Convert[me: i32]() -> i32 { return me; }
+}
+
+fn add[T:! IntLike](x: {.a: T, .b: ({.m: i32, .n: T}, i32)}) -> i32 {
+  return 1000 * x.a.Convert() + 100 * x.b[0].m + 10 * x.b[0].n.Convert() + x.b[1];
+}
+
+fn Main() -> i32 {
+  return add({.b = ({.n = 1, .m = 2}, 3), .a = 4});
+}