zig: add build-pch command to emit precompiled C header.

usage example:
        `zig build-pch -lc++ -x c++-header test.h`
        `zig run -lc++ -cflags -include-pch test.pch -- main.cpp`

It builds the file.pch with llvm "-fpch-validate-input-files-content",
so it includes data for better integration with zig caching system.

src/Compilation.zig

@@ -4571,11 +4571,11 @@ fn updateCObject(comp: *Compilation, c_object: *CObject, c_obj_prog_node: *std.P
             else
                 "/dev/null";
 
-            try argv.ensureUnusedCapacity(6);
+            try argv.ensureUnusedCapacity(7);
             switch (comp.clang_preprocessor_mode) {
                 .no => argv.appendSliceAssumeCapacity(&.{ "-c", "-o", out_obj_path }),
                 .yes => argv.appendSliceAssumeCapacity(&.{ "-E", "-o", out_obj_path }),
-                .pch => argv.appendSliceAssumeCapacity(&.{ "-Xclang", "-emit-pch", "-o", out_obj_path }),
+                .pch => argv.appendSliceAssumeCapacity(&.{ "-Xclang", "-emit-pch", "-fpch-validate-input-files-content", "-o", out_obj_path }),
                 .stdout => argv.appendAssumeCapacity("-E"),
             }
 
@@ -4610,11 +4610,11 @@ fn updateCObject(comp: *Compilation, c_object: *CObject, c_obj_prog_node: *std.P
         try argv.appendSlice(c_object.src.extra_flags);
         try argv.appendSlice(c_object.src.cache_exempt_flags);
 
-        try argv.ensureUnusedCapacity(6);
+        try argv.ensureUnusedCapacity(7);
         switch (comp.clang_preprocessor_mode) {
             .no => argv.appendSliceAssumeCapacity(&.{ "-c", "-o", out_obj_path }),
             .yes => argv.appendSliceAssumeCapacity(&.{ "-E", "-o", out_obj_path }),
-            .pch => argv.appendSliceAssumeCapacity(&.{ "-Xclang", "-emit-pch", "-o", out_obj_path }),
+            .pch => argv.appendSliceAssumeCapacity(&.{ "-Xclang", "-emit-pch", "-fpch-validate-input-files-content", "-o", out_obj_path }),
             .stdout => argv.appendAssumeCapacity("-E"),
         }
         if (comp.clang_passthrough_mode) {

src/main.zig

@@ -80,6 +80,7 @@ const normal_usage =
     \\  build-exe        Create executable from source or object files
     \\  build-lib        Create library from source or object files
     \\  build-obj        Create object from source or object files
+    \\  build-pch        Create a precompiled header from a c or c++ header
     \\  test             Perform unit testing
     \\  run              Create executable and run immediately
     \\
@@ -262,6 +263,8 @@ fn mainArgs(gpa: Allocator, arena: Allocator, args: []const []const u8) !void {
         return buildOutputType(gpa, arena, args, .{ .build = .Lib });
     } else if (mem.eql(u8, cmd, "build-obj")) {
         return buildOutputType(gpa, arena, args, .{ .build = .Obj });
+    } else if (mem.eql(u8, cmd, "build-pch")) {
+        return buildOutputType(gpa, arena, args, .pch);
     } else if (mem.eql(u8, cmd, "test")) {
         return buildOutputType(gpa, arena, args, .zig_test);
     } else if (mem.eql(u8, cmd, "run")) {
@@ -366,6 +369,7 @@ const usage_build_generic =
     \\Usage: zig build-exe   [options] [files]
     \\       zig build-lib   [options] [files]
     \\       zig build-obj   [options] [files]
+    \\       zig build-pch   [options] [files]
     \\       zig test        [options] [files]
     \\       zig run         [options] [files] [-- [args]]
     \\       zig translate-c [options] [file]
@@ -712,6 +716,7 @@ const ArgMode = union(enum) {
     build: std.builtin.OutputMode,
     cc,
     cpp,
+    pch,
     translate_c,
     zig_test,
     run,
@@ -997,11 +1002,15 @@ fn buildOutputType(
     var color: Color = if (native_os == .wasi or EnvVar.NO_COLOR.isSet()) .off else .auto;
 
     switch (arg_mode) {
-        .build, .translate_c, .zig_test, .run => {
+        .build, .translate_c, .zig_test, .run, .pch => {
             switch (arg_mode) {
                 .build => |m| {
                     create_module.opts.output_mode = m;
                 },
+                .pch => {
+                    create_module.opts.output_mode = .Obj;
+                    clang_preprocessor_mode = .pch;
+                },
                 .translate_c => {
                     emit_bin = .no;
                     create_module.opts.output_mode = .Obj;