[GWP-ASan] Respect GWP_ASAN_DEFAULT_ENABLED compile-time macro

If the containing allocator build uses -DGWP_ASAN_DEFAULT_ENABLED=false
then the option will default to false.  For e.g. Scudo, this is simpler
and more efficient than using -DSCUDO_DEFAULT_OPTIONS=... to set gwp-asan
options that have to be parsed from the string at startup.

Reviewed By: hctim

Differential Revision: https://reviews.llvm.org/D91463