Full response file (*.rsp) support

I hit the "quotes in an RSP file" issue when trying to compile gRPC using
"zig cc". As a fun exercise, I decided to see if I could fix it myself.
I'm fully open to this code being flat-out rejected. Or I can take feedback
to fix it up.

This modifies (and renames) _ArgIteratorWindows_ in process.zig such that
it works with arbitrary strings (or the contents of an RSP file).

In main.zig, this new _ArgIteratorGeneral_ is used to address the "TODO"
listed in _ClangArgIterator_.

This change closes #4833.

**Pros:**

- It has the nice attribute of handling "RSP file" arguments in the same way it
  handles "cmd_line" arguments.
- High Performance, minimal allocations
- Fixed bug in previous _ArgIteratorWindows_, where final trailing backslashes
  in a command line were entirely dropped
- Added a test case for the above bug
- Harmonized the _ArgIteratorXxxx._initWithAllocator()_ and _next()_ interface
  across Windows/Posix/Wasi (Moved Windows errors to _initWithAllocator()_
  rather than _next()_)
- Likely perf benefit on Windows by doing _utf16leToUtf8AllocZ()_ only once
  for the entire cmd_line

**Cons:**

- Breaking Change in std library on Windows: Call
  _ArgIterator.initWithAllocator()_ instead of _ArgIterator.init()_
- PhaseMage is new with contributions to Zig, might need a lot of hand-holding
- PhaseMage is a Windows person, non-Windows stuff will need to be double-checked

**Testing Done:**

- Wrote a few new test cases in process.zig
- zig.exe build test -Dskip-release (no new failures seen)
- zig cc now builds gRPC without error

1 files changed, 4 insertions, 5 deletions

diff --git a/test/cli.zig b/test/cli.zig
index c99c86b008..5df5696919 100644
--- a/test/cli.zig
+++ b/test/cli.zig
@@ -11,18 +11,17 @@ pub fn main() !void {
     var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
     defer arena.deinit();
 
-    var arg_it = process.args();
+    a = arena.allocator();
+    var arg_it = try process.argsWithAllocator(a);
 
     // skip my own exe name
     _ = arg_it.skip();
 
-    a = arena.allocator();
-
-    const zig_exe_rel = (try arg_it.next(a)) orelse {
+    const zig_exe_rel = arg_it.next() orelse {
         std.debug.print("Expected first argument to be path to zig compiler\n", .{});
         return error.InvalidArgs;
     };
-    const cache_root = (try arg_it.next(a)) orelse {
+    const cache_root = arg_it.next() orelse {
         std.debug.print("Expected second argument to be cache root directory path\n", .{});
         return error.InvalidArgs;
     };