Add flag for root cause diagnostic output

[john-h-kastner]

This pull requests adds a command line flag to print the root cause of wildness information that was previously being used in the clangd interactive mode.

Example usage: cconv-standalone -alltypes -warn-root-cause fptrarrstructcallee.c > /dev/null

Error while trying to load a compilation database:
Could not auto-detect compilation database for file "fptrarrstructcallee.c"
No compilation database found in /home/cc/checkedc-clang/clang/test/CheckedCRewriter or any parent directory
fixed-compilation-database: Error while opening fixed database: No such file or directory
json-compilation-database: Error while opening JSON database: No such file or directory
Running without flags.
/home/cc/checkedc-clang/clang/test/CheckedCRewriter/fptrarrstructcallee.c:27:48: warning: Root cause of unchecked pointers: Inner pointer of a parameter to external function.
extern _Unchecked char *strcpy(char * restrict dest, const char * restrict src : itype(restrict _Nt_array_ptr<const char>));
                                               ^
/home/cc/checkedc-clang/clang/test/CheckedCRewriter/fptrarrstructcallee.c:27:25: warning: Root cause of unchecked pointers: Return value of an external function:strcpy
extern _Unchecked char *strcpy(char * restrict dest, const char * restrict src : itype(restrict _Nt_array_ptr<const char>));
                        ^
/home/cc/checkedc-clang/clang/test/CheckedCRewriter/fptrarrstructcallee.c:108:13: warning: Root cause of unchecked pointers: Cast from int to struct fptrarr *
        x = (struct fptrarr *) 5; 
            ^
3 warnings generated.

The default output with -warn-root-cause filters out root causes located in header files that are only responsible for one wild pointer. If you want to see all root causes, I've also added -warn-all-root-cause to disable this filtering.

I've also made two tweaks to constraint generation that improve the root cause output:

1. Invalid casts should be reported one time instead of once at each pointer level
2. Void pointer were being reported as var arg arrays.

[mwhicks1]

If you are in the main file then you will output the warning even if there's just one wild pointer?

[john-h-kastner]

In this case main file is per translation unit, so for converting vsftpd, the main files are all the c files passed in as command line arguments. The idea is that all wild pointers in these files are interesting whereas a wild pointer in an included header file that doesn't cause wildness anywhere else is generally not interesting.

[mwhicks1]

Looks great! Merge when ready (only comments were about comments).