Warn instead of fail when swiftc not found on Linux (#1433)

Signed-off-by: Brentley Jones <github@brentleyjones.com>
(cherry picked from commit a8d7635)

swift/internal/swift_autoconfiguration.bzl

@@ -208,7 +208,18 @@ def _create_linux_toolchain(repository_ctx):
     """
     path_to_swiftc = repository_ctx.which("swiftc")
     if not path_to_swiftc:
-        fail("No 'swiftc' executable found in $PATH")
+        print("""\
+No 'swiftc' executable found in $PATH. Not auto-generating a Linux Swift \
+toolchain.
+""")  # buildifier: disable=print
+        repository_ctx.file(
+            "BUILD",
+            """\
+# No 'swiftc' executable found in $PATH. Not auto-generating a Linux Swift \
+toolchain.
+""",
+        )
+        return
 
     root = path_to_swiftc.dirname.dirname
     feature_values = _compute_feature_values(repository_ctx, path_to_swiftc)
@@ -223,7 +234,7 @@ def _create_linux_toolchain(repository_ctx):
 
     repository_ctx.file(
         "BUILD",
-        """
+        """\
 load(
     "@build_bazel_rules_swift//swift/toolchains:swift_toolchain.bzl",
     "swift_toolchain",