[build-tooling-libs] Add option to specify the swiftc compiler to build compiler sources

This is needed to build the most recent state of main if it uses C++ interop features that aren't in the host's toolchain yet.

Author: ahoppen

utils/build-tooling-libs

@@ -67,6 +67,7 @@ class Builder(object):
         self.host = host
         self.arch = arch
         self.native_build_dir = native_build_dir
+        self.swiftc = args.swiftc or self.toolchain.swiftc
 
     def call(self, command, env=None, without_sleeping=False):
         if without_sleeping:
@@ -172,7 +173,7 @@ class Builder(object):
             "-DSWIFT_HOST_VARIANT_SDK=" + host_sdk,
             "-DSWIFT_HOST_VARIANT_ARCH=" + self.arch,
             "-DCMAKE_Swift_COMPILER_TARGET=" + host_triple,
-            "-DCMAKE_Swift_COMPILER=" + self.toolchain.swiftc,
+            "-DCMAKE_Swift_COMPILER=" + self.swiftc,
             "-DCMAKE_C_FLAGS=" + llvm_c_flags,
             "-DCMAKE_CXX_FLAGS=" + llvm_c_flags,
         ]
@@ -499,6 +500,16 @@ Example invocations:
         default=default_ninja,
         help="the path to ninja (default = %s)" % default_ninja,
     )
+    option(
+        "--swiftc",
+        store_path,
+        default=None,
+        help="""
+        Override the swiftc compiler used to build SwiftCompilerSources.
+        This is needed to build the most recent state of main if it uses C++ interop 
+        features that aren't in the host's toolchain yet.
+        """
+    )
 
     parser = optbuilder.build()
     args = parser.parse_args()