Enable -Wctad-maybe-unsupported in LLVM build

Warns on potentially unintended use of C++17 Class Template Argument
Deduction. Use of this feature with types that aren't intended to
support it may may future refactorings of those types more difficult -
so this warning fires whenever the feature is used with a type that may
not have intended to be used with CTAD (the warning uses the existence
of at least one explicit deduction guide to indicate that a type
intentionally supports CTAD - absent that, it's assumed to not be
intended to support CTAD & produces a warning).

This is disabled in libcxx because lots of the standard library is
assumed to provide ctad-usable APIs and the false positive suppression
in the diagnostic is based on system header classification which doesn't
apply in the libcxx build itself.

Differential Revision: https://reviews.llvm.org/D131727