[bootstrap] Preemptively add argument for Foundation

swiftlang/swift-corelibs-xctest#43 will
add a dependency between swift-corelibs-xctest and
swift-corelibs-foundation. This will necessitate the path to
a Foundation build be passed to the SwiftPM bootstrap script.

In order to prevent CI from breaking, we'll modify the Swift
build script to pass the correct parameters *before* those
parameters are actually used. To do that, this commit ensures
the bootstrap script accepts the new parameter: "--foundation".

Author: modocache

Utilities/bootstrap

@@ -449,6 +449,8 @@ def main():
                         default="/usr/local", metavar="PATH")
     parser.add_argument("-v", "--verbose", action="store_true",
                         help="use verbose output")
+    parser.add_argument("--foundation", dest="foundation_path",
+                        help="Path to Foundation build directory")
     parser.add_argument("--xctest", dest="xctest_path",
                         help="Path to XCTest build directory")
     parser.add_argument("--build-tests", action="store_true",