Compiler stages: model "meta" stages

[woodruffw]

Clang has a bunch of flags that cause the clang and clang++ drivers to print out information, rather than actually doing anything with inputs.

We handle some of these already (like -### and -v) by marking them as CompilerStage.Unknown, but we should really mark them as something like CompilerStage.Meta and handle other common cases:

• -print-file-name=<file>
• -print-libgcc-file-name
• -print-prog-name=<prog>
• -print-search-dirs

Each of the above also works as a "long" option, e.g. --print-search-dirs. GCC also probably has more of its own.