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.

llvm-svn: 314735