PATCH: Add --with-build-config=bootstrap-asan support

[Konstantin Serebryany]

> Does -faddress-sanitizer add a CPP symbol that lets you work around it?
>  If not, it's a bug.  If so, please make libcpp revert to the dump
> algorithm if said symbol is defined.

Do you mean, does asan compilation cause some special preprocessor
symbol to be defined?
No, asan  compilation does not define any preprocessor symbol.
In clang, there are two ways to workaround asan-hostile code: one is a
function attribute and another is __has_feature macro
http://clang.llvm.org/docs/AddressSanitizer.html#has_feature
Both are clang-specific today, but both make sense in gcc as well.

--kcc