Macro expander fails on preprocessor directives inside block comment in `bh`

[mattmccutchen-cci]

bh (in Olden) has a block comment containing preprocessor directives:

/********
#define global extern
#include "defs.h"
#include "code.h"
#include "mem-ref.h"
*********/

The preprocessor removes the entire block comment, but then the macro expander adds back the directives as it normally does: it's not smart enough to realize that they were commented out. This causes a verification failure because the #include directives point to files that do not exist in the current version of bh: presumably that's why they had been commented out!

A possible workaround is to have the macro expander use the -C option to clang -E to preserve comments. It still won't know any better than to think that the directives are real directives that it should force to be preserved, but the preprocessor will keep the /* ... */ around them, so it won't matter. This is starting to illustrate the design fragility of the macro expander: while adding -C sets off a chain of events that leads to the desired end result in this case, it equally well might break other edge cases. Fortunately, the built-in verification ensures that we are notified of any problem right away rather than silently getting wrong results, so maybe I can just test the change on the benchmark workflow and we'll be satisfied if that passes.

Other benefits of adding -C:

• It will preserve the /**/ comments that we added to some benchmarks to work around correctcomputation/C3#10, so we'll no longer have to patch those comments using sed in the analyse-conv makefile.
• In general, keeping comments probably makes things nicer for the user if they have to read the macro-expanded code for troubleshooting and by keeping irrelevant comment removals out of the diffs the user has to review during the proposed textual merge workflow.