[sanitizer] Do not mmap FlagParser::flags_

Instead, make it a static array that's part of the FlagParser. The advantage
this has is helping reduce fragmentation from needing to anonymously mmap
this array via the LowLevelAllocator. This will instead place the array on
the stack. Functionally, the only difference is that the array will not be
zero-initialized, but all used elements are explicitly initialized via the
flag handlers.

Differential Revision: https://reviews.llvm.org/D158780