[ABI] Handle C++20 [[no_unique_address]] attribute

Many platform ABIs have special support for passing aggregates that
either just contain a single member of floatint-point type, or else
a homogeneous set of members of the same floating-point type.

When making this determination, any extra "empty" members of the
aggregate type will typically be ignored.  However, in C++ (at least
in all prior versions), no data member would actually count as empty,
even if it's type is an empty record -- it would still be considered
to take up at least one byte of space, and therefore make those ABI
special cases not apply.

This is now changing in C++20, which introduced the [[no_unique_address]]
attribute.  Members of empty record type, if they also carry this
attribute, now do *not* take up any space in the type, and therefore
the ABI special cases for single-element or homogeneous aggregates
should apply.

The C++ Itanium ABI has been updated accordingly, and GCC 10 has
added support for this new case.  This patch now adds support to
LLVM.  This is cross-platform; it affects all platforms that use
the single-element or homogeneous aggregate ABI special case and
implement this using any of the following common subroutines
in lib/CodeGen/TargetInfo.cpp:
  isEmptyField
  isEmptyRecord
  isSingleElementStruct
  isHomogeneousAggregate

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