c/c++: Tweak warning for 'always_inline function might not be inlinable'

When you're not regularly exposed to this warning, it is
easy to be misled by its wording, believing that there's
something else in the function that stops it from being
inlined, something other than the lack of also being
*declared* inline.  Also, clang does not warn.

It's just a warning: without the inline directive, there has
to be a secondary reason for the function to be inlined,
other than the always_inline attribute, a reason that may be
in effect despite the warning.

Whenever the text is quoted in inline-related bugzilla
entries, there seems to often have been an initial step of
confusion that has to be cleared, for example in PR55830.
A file in the powerpc-specific parts of the test-suite,
gcc.target/powerpc/vec-extract-v16qiu-v2.h, has a comment
and seems to be another example, and I testify as the
first-hand third "experience".  The wording has been the
same since the warning was added.

Let's just tweak the wording, adding the cause, so that the
reason for the warning is clearer.  This hopefully stops the
user from immediately asking "'Might'?  Because why?"  and
then going off looking at the function body - or grepping
the gcc source or documentation, or enter a bug-report
subsequently closed as resolved/invalid.

Since the message is only appended with additional
information, no test-case actually required adjustment.
I still changed them, so the message is covered.

gcc:
	* cgraphunit.cc (process_function_and_variable_attributes): Tweak
	the warning for an attribute-always_inline without inline declaration.

gcc/testsuite:
	* g++.dg/Wattributes-3.C: Adjust expected warning.
	* gcc.dg/fail_always_inline.c: Ditto.