BUILD.gn: introduce finer grained internal targets for Tint (#4399)

Tint reaches out into SPIRV-Tools BUILD.gn file for some targets:

 - Adds spvtools_include_gen_dirs so that Tint can add $target_gen_dir
   and find autogenerated files.
 - Adds spvtools_language_headers that Tint can reference so it
   automatically picks up new language headers that are added.
diff --git a/BUILD.gn b/BUILD.gn
index 791d7a3..fdf4584 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -358,14 +358,20 @@
   include_dirs = [ "include" ]
 }
 
+config("spvtools_include_gen_dirs") {
+  include_dirs = [ "$target_gen_dir" ]
+}
+
 config("spvtools_internal_config") {
   include_dirs = [
     ".",
-    "$target_gen_dir",
     "${spirv_headers}/include",
   ]
 
-  configs = [ ":spvtools_public_config" ]
+  configs = [
+    ":spvtools_public_config",
+    ":spvtools_include_gen_dirs",
+  ]
 
   cflags = []
   if (is_clang) {
@@ -391,6 +397,14 @@
   public_configs = [ ":spvtools_public_config" ]
 }
 
+group("spvtools_language_headers") {
+  public_deps = [
+    ":spvtools_language_header_cldebuginfo100",
+    ":spvtools_language_header_debuginfo",
+    ":spvtools_language_header_vkdebuginfo100",
+  ]
+}
+
 static_library("spvtools") {
   deps = [
     ":spvtools_core_tables_unified1",