preprocessor: Fix pp-number lexing of digit separators [PR83873, PR97604]

When the preprocessor lexes preprocessing numbers in lex_number, it
accepts digit separators in more cases than actually permitted in
pp-numbers by the standard syntax.

One thing this accepts is adjacent digit separators; there is some
code to reject those later, but as noted in bug 83873 it fails to
cover the case of adjacent digit separators within a floating-point
exponent.  Accepting adjacent digit separators only results in a
missing diagnostic, not in valid code being rejected or being accepted
with incorrect semantics, because the correct lexing in such a case
would have '' start the following preprocessing tokens, and no valid
preprocessing token starts '' while ' isn't valid on its own as a
preprocessing token either.  So this patch fixes that case by moving
the error for adjacent digit separators to lex_number (allowing a more
specific diagnostic than if '' were excluded from the pp-number
completely).

Other cases inappropriately accepted involve digit separators before
'.', 'e+', 'e-', 'p+' or 'p-' (or corresponding uppercase variants).
In those cases, as shown by the test digit-sep-pp-number.C added, this
can result in valid code being wrongly rejected as a result of too
many characters being included in the pp-number.  So this case is
fixed by terminating the pp-number at the correct character according
to the standard.  That test also covers the case where a digit
separator was followed by an identifier-nondigit that is not a
nondigit (e.g. a UCN); that case was already handled correctly.

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

libcpp/
	PR c++/83873
	PR preprocessor/97604
	* lex.c (lex_number): Reject adjacent digit separators here.  Do
	not allow digit separators before '.' or an exponent with sign.
	* expr.c (cpp_classify_number): Do not check for adjacent digit
	separators here.

gcc/testsuite/
	PR c++/83873
	PR preprocessor/97604
	* g++.dg/cpp1y/digit-sep-neg-2.C,
	g++.dg/cpp1y/digit-sep-pp-number.C: New tests.
	* g++.dg/cpp1y/digit-sep-line-neg.C, g++.dg/cpp1y/digit-sep-neg.C:
	Adjust expected messages.