When using a default function argument for a function template (or

member function thereof), perform the template instantiation each time
the default argument is needed. This ensures that
  (1) We get different CXXTemporary objects for each instantiation, and
  (2) Any other instantiations or definitions triggered by the
  instantiation of the default argument expression are guaranteed to
  happen; previously, they might have been suppressed, e.g., because
  they happened in an unevaluated context.

This fixes the majority of PR5810. However, it does not address the
problem where we may have multiple uses of the same CXXTemporary
within an expression when the temporary came from a non-instantiated
default argument expression.

llvm-svn: 92015