[lit] Allow overriding each individual command.

Author: ddunbar

lit.cfg

@@ -151,13 +151,19 @@ if os.path.exists(swiftpm_srcdir):
 # Find the tools we need.
 lit_config.note("testing using 'FileCheck': %r" % (filecheck_path,))
 
-swift_path = os.path.join(package_path, "usr", "bin", "swift")
+swift_path = lit_config.params.get(
+    "swift",
+    os.path.join(package_path, "usr", "bin", "swift"))
 lit_config.note("testing using 'swift': %r" % (swift_path,))
 
-swiftc_path = os.path.join(package_path, "usr", "bin", "swiftc")
+swiftc_path = lit_config.params.get(
+    "swiftc",
+    os.path.join(package_path, "usr", "bin", "swiftc"))
 lit_config.note("testing using 'swiftc': %r" % (swiftc_path,))
 
-lldb_path = os.path.join(package_path, "usr", "bin", "lldb")
+lldb_path = lit_config.params.get(
+    "lldb",
+    os.path.join(package_path, "usr", "bin", "lldb"))
 lit_config.note("testing using 'lldb': {}".format(lldb_path))
 
 # Verify they exist.