[pstl] Allow customizing whether per-TU insulation is provided

Like we do in libc++, PSTL needs the ability to constrain
ABI-unstable symbols to each translation unit. This is OFF by
default (like for libc++), because most people don't care about
this and there is a cost associated to enabling the option (code
bloat because templates are not deduped across TUs).

I'm using '#pragma clang attribute push' to avoid marking each
declaration with an attribute, which quickly becomes difficult
to maintain.

llvm-svn: 368684