Improve test runner output for broken configurations.

Previously LIT would often fail while attempting to set up/configure
the test compiler; normally when attempting to dump the builtin macros.
This sort of failure provided no useful information about what went
wrong with the compiler, making the actual issues hard --- if not
impossible --- to debug easily.

This patch changes the LIT configuration to report the failure explicitly,
including the failed compile command and the stdout/stderr output.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@314735 91177308-0d34-0410-b5e6-96231b3b80d8

utils/libcxx/compiler.py

@@ -204,7 +204,7 @@ class CXXCompiler(object):
         flags = ['-dM'] + flags
         cmd, out, err, rc = self.preprocess(source_files, flags=flags, cwd=cwd)
         if rc != 0:
-            return None
+            return cmd, out, err, rc
         parsed_macros = {}
         lines = [l.strip() for l in out.split('\n') if l.strip()]
         for l in lines: