Use \external:I instead of -isystem when using MSVC for header dirs

Summary:
WTTS
This allows us to correctly include header dirs and build bigger projects

Reviewed By: ndmitchell

Differential Revision: D44956819

fbshipit-source-id: b1ff91f9169562388d33e8c9e8c91924cf5b3971

cxx/cxx.bzl

@@ -116,6 +116,7 @@ load(
     "cxx_exported_preprocessor_info",
     "cxx_inherited_preprocessor_infos",
     "cxx_merge_cpreprocessors",
+    "format_system_include_arg",
 )
 
 cxx_link_into_shared_library = _cxx_link_into_shared_library
@@ -340,9 +341,10 @@ def prebuilt_cxx_library_impl(ctx: "context") -> ["provider"]:
     inherited_pp_infos = cxx_inherited_preprocessor_infos(exported_first_order_deps)
     generic_exported_pre = cxx_exported_preprocessor_info(ctx, cxx_get_regular_cxx_headers_layout(ctx), [])
     args = cxx_attr_exported_preprocessor_flags(ctx)
+    compiler_type = get_cxx_toolchain_info(ctx).cxx_compiler_info.compiler_type
     if header_dirs != None:
         for x in header_dirs:
-            args += ["-isystem", x]
+            args.append(format_system_include_arg(cmd_args(x), compiler_type))
     specific_exportd_pre = CPreprocessor(args = args)
     providers.append(cxx_merge_cpreprocessors(
         ctx,

cxx/preprocessor.bzl

@@ -82,7 +82,7 @@ def _cpreprocessor_include_dirs(pres: [CPreprocessor.type]):
             args.add(cmd_args(d, format = "-I{}"))
         if pre.system_include_dirs != None:
             for d in pre.system_include_dirs.include_dirs:
-                system_include_args = _format_system_include_arg(d, pre.system_include_dirs.compiler_type)
+                system_include_args = format_system_include_arg(cmd_args(d), pre.system_include_dirs.compiler_type)
                 args.add(system_include_args)
     return args
 
@@ -161,11 +161,11 @@ def _format_include_arg(flag: str.type, path: "cmd_args", compiler_type: str.typ
     else:
         return [cmd_args(flag), path]
 
-def _format_system_include_arg(path: "label_relative_path", compiler_type: str.type) -> ["cmd_args"]:
+def format_system_include_arg(path: "cmd_args", compiler_type: str.type) -> ["cmd_args"]:
     if compiler_type == "windows":
         return [cmd_args(path, format = "/external:I{}")]
     else:
-        return [cmd_args("-isystem"), cmd_args(path)]
+        return [cmd_args("-isystem"), path]
 
 def cxx_exported_preprocessor_info(ctx: "context", headers_layout: CxxHeadersLayout.type, extra_preprocessors: [CPreprocessor.type] = []) -> CPreprocessor.type:
     """