[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

README.md

@@ -84,7 +84,12 @@ Tests are an important part of the development and evolution of this project,
 and new contributions are expected to include tests for any functionality
 change.  To run the tests, pass the `test` verb to the `bootstrap` script:
 
-    ./Utilities/bootstrap --build-tests test
+    ./Utilities/bootstrap \
+        --swiftc /path/to/swift-linux-x86_64/bin/swiftc \
+        --sbt /path/to/llbuild-linux-x86_64/bin/swift-build-tool \
+        --foundation /path/to/foundation-linux-x86_64/Foundation \
+        --xctest /path/to/xctest-linux-x86_64 \
+        test
 
 > Long-term, we intend for testing to be an integral part of the Package Manager itself
 > and to not require custom support.

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",