add asan_ignore - which errors to ignore

"clang -fsanitize=undefined" produces lots of false positives.
This file allows certain functions to be excluded from checking.
Use it as:

    clang -fsanitize=undefined -fsanitize-blacklist=`pwd`/asan_ignore
fix integer overflow in S_scan_commit().

offset_float_max could end up as SSize_t_MAX+1.

This triggered it: /^x(ab|c.+)$/.

Found by -fsanitize=undefined.
fix integer overflow in S_study_chunk().

It was adding SSize_t_MAX to data->last_start_max when
data->last_start_max was already SSize_t_MAX.

This triggered it: /(x+y)+/.

Found by -fsanitize=undefined.
fix integer overflow in S_study_chunk().

It was adding to delta even when delta was already SSize_t_MAX

This triggered it: /.*(ab|abc)/.

Found by -fsanitize=undefined.
fix integer overflow in S_regpiece().

RExC_naughty is incremented when nasty bits of regex are found.
If at the end of compilation, its > 01, then PREGf_NAUGHTY is set on the
pattern.

However, some bits of S_regpiece on detecting naughiness, double or nearly
double RExC_naughty, quickly resulting in overflow of the I32.

I've fixed it by skipping the doubling when RExC_naughty is large,
but I don't know whether the doubling is conceptually wrong in the first
place.

Found by -fsanitize=undefined.