Pass new test library directory to builds

With Xcode 11 there is a new location where there are Swift specific
libraries and swiftmodules. Specifically they contain the new
`XCTUnwrap` function. This change passes these arguments to the compiler
and the linker.

For iOS tests this also requires bazelbuild/rules_apple#572

Fixes #266

swift/internal/xcode_swift_toolchain.bzl

@@ -22,6 +22,7 @@ toolchain, see `swift.bzl`.
 load("@bazel_skylib//lib:collections.bzl", "collections")
 load("@bazel_skylib//lib:dicts.bzl", "dicts")
 load("@bazel_skylib//lib:partial.bzl", "partial")
+load("@bazel_skylib//lib:paths.bzl", "paths")
 load("@bazel_tools//tools/cpp:toolchain_utils.bzl", "find_cpp_toolchain")
 load(
     ":features.bzl",
@@ -35,6 +36,17 @@ load(
 )
 load(":providers.bzl", "SwiftToolchainInfo")
 
+def _swift_developer_lib_dir(platform_framework_dir):
+    """Returns the directory contain extra Swift specific developer libraries
+
+    Args:
+        platform_framework_dir: The developer platform framework directory for the current platform
+
+    Returns:
+        The directory containing Swift specific development libraries and swiftmodules
+    """
+    return paths.join(paths.dirname(paths.dirname(platform_framework_dir)), "usr", "lib")
+
 def _command_line_objc_copts(objc_fragment):
     """Returns copts that should be passed to `clang` from the `objc` fragment.
 
@@ -140,6 +152,7 @@ def _default_linker_opts(
     # their rpaths.
     if is_test:
         linkopts.append("-Wl,-rpath,{}".format(platform_framework_dir))
+        linkopts.append("-L{}".format(_swift_developer_lib_dir(platform_framework_dir))
 
     return linkopts
 
@@ -154,13 +167,16 @@ def _default_swiftc_copts(apple_fragment, apple_toolchain, target):
     Returns:
         A list of options that will be passed to any compile action created by this toolchain.
     """
+    platform_framework_dir = apple_toolchain.platform_developer_framework_dir(apple_fragment)
     copts = [
         "-target",
         target,
         "-sdk",
         apple_toolchain.sdk_dir(),
         "-F",
-        apple_toolchain.platform_developer_framework_dir(apple_fragment),
+        platform_framework_dir,
+        "-I",
+        _swift_developer_lib_dir(platform_framework_dir),
     ]
 
     bitcode_mode = str(apple_fragment.bitcode_mode)