Fix diagnostics for never-defined inline and nested functions (PR c/88720, PR c/88726).

Fix diagnostics for never-defined inline and nested functions (PR c/88720, PR c/88726).

Bugs 88720 and 88726 report issues where a function is declared inline
in an inner scope, resulting in spurious diagnostics about it being
declared but never defined when that scope is left (possibly in some
cases also wrongly referring to the function as a nested function).
These are regressions that were introduced with the support for C99
inline semantics in 4.3 (they don't appear with 4.2; it's possible
some aspects of the bugs might have been introduced later than 4.3).

For the case of functions being wrongly referred to as nested,
DECL_EXTERNAL was not the right condition for a function being
non-nested; TREE_PUBLIC is appropriate for the case of non-nested
functions with external linkage, while !b->nested means this is the
outermost scope in which the function was declared and so avoids
catching the case of a file-scope static being redeclared inline
inside a function.

For the non-nested, external-linkage case, the code attempts to avoid
duplicate diagnostics by diagnosing only when scope != external_scope,
but actually scope == external_scope is more appropriate, as it's only
when the file and external scopes are popped that the code can
actually tell whether a function ended up being defined, and all such
functions will appear in the (GCC-internal) external scope.

Bootstrapped with no regressions on x86_64-pc-linux-gnu.

	PR c/88720
	PR c/88726
gcc/c:
	* c-decl.c (pop_scope): Use TREE_PUBLIC and b->nested to determine
	whether a function is nested, not DECL_EXTERNAL.  Diagnose inline
	functions declared but never defined only for external scope, not
	for other scopes.

gcc/testsuite:
	* gcc.dg/inline-40.c, gcc.dg/inline-41.c: New tests.

From-SVN: r267665