[lit] Run cpp tests.

Author: nate-chandler

test/lit.cfg

@@ -193,7 +193,7 @@ config.test_format = swift_test.SwiftTest(coverage_mode=config.coverage_mode,
 
 # suffixes: A list of file extensions to treat as test files.
 config.suffixes = ['.swift', '.ll', '.sil', '.gyb', '.m', '.c',
-                   '.swiftinterface', '.test-sh', '.test']
+                   '.swiftinterface', '.test-sh', '.test', '.cpp']
 
 # excludes: A list of directories to exclude from the testsuite. The 'Inputs'
 # subdirectories contain auxiliary inputs for various tests in their parent