[analyzer] Consider single-elem arrays as FAMs by default

According to my measurement in https://reviews.llvm.org/D108230#3933232,
it seems like there is no drawback to enabling this analyzer-config by default.

Actually, enabling this by default would make it consistent with the
codegen of clang, which according to `-fstrict-flex-arrays`, assumes
by default that all trailing arrays could be FAMs, let them be of size
undefined, zero, one, or anything else.

Speaking of `-fstrict-flex-arrays`, in the next patch I'll deprecate
the analyzer-config FAM option in favor of that flag. That way, CSA will
always be in sync with what the codegen will think of FAMs.

So, if a new codebase sets `-fstrict-flex-arrays` to some value above 0,
CSA will also make sure that only arrays of the right size will be
considered as FAMs.

Reviewed By: xazax.hun

Differential Revision: https://reviews.llvm.org/D138657