Get the realpath of the build directory when building SwiftSyntax

We were currently canonicalizing the build directory when testing SwiftSyntax but not when building it. This caused errors if the build directory was a symlink because module caches were created for the non-canonicalized path during the build and then referenced with the canonicalized path during testing.

Author: ahoppen

build-script.py

@@ -851,7 +851,7 @@ def main():
     try:
         builder = Builder(
             toolchain=args.toolchain,
-            build_dir=args.build_dir,
+            build_dir=realpath(args.build_dir),
             multiroot_data_file=args.multiroot_data_file,
             release=args.release,
             verbose=args.verbose,