[clang] CWG 2082 and 2346: loosen the restrictions on parameters and local...

[clang] CWG 2082 and 2346: loosen the restrictions on parameters and local variables in default arguments.

This patch implements the resolution of CWG 2082 and CWG 2346.

The resolution of CWG 2082 changed [dcl.fct.default]p7 and p9 to allow
a parameter or local variable to appear in a default argument if not
in a potentially-evaluated expression.

The resolution of CWG 2346 changed [dcl.fct.default]p7 to allow a local
variable to appear in a default argument if not odr-used.

An issue remains after this patch
(see the FIXME in test/CXX/dcl.decl/dcl.meaning/dcl.fct.default/p7.cpp).
This is addressed by the next patch.

Differential Revision: https://reviews.llvm.org/D81615

Reviewed By: rsmith, erichkeane