W/ Clang, compile C/C++ testcases with -Wno-unknown-warning-option

Some C/C++ testcases unconditionally pass -Wno-foo as additional
options to disable some warning.  That is OK with GCC, because GCC
accepts -Wno-foo silently even if it doesn't support -Wfoo.  This is a
feature which allows disabling warnings with newer compilers without
breaking builds with older compilers.  Clang however warns about
unknown -Wno-foo by default, unless you pass
-Wno-unknown-warning-option as well:

 $ gcc -Wno-foo test.c
 * nothing, compiles successfuly *

 $ clang -Wno-foo test.c
 warning: unknown warning option '-Wno-foo [-Wunknown-warning-option]

This commit adds -Wunknown-warning-option centrally in gdb_compile, so
that individual testcases don't have to worry about breaking older
Clangs.

IOW, this avoids this problematic scenario:

#1 - A testcase compiles successfully with Clang version X.
#2 - Clang version "X + 1" adds a new warning, enabled by default,
     which breaks the test.
#3 - We add -Wno-newwarning to the testcase, fixing the testcase with
     clang "X + 1".
#4 - Now building the test with Clang version X no longer works, due
     to "unknown warning option".

gdb/testsuite/ChangeLog:
2020-06-24  Pedro Alves  <palves@redhat.com>

	* lib/gdb.exp (gdb_compile): Update intro comment.  If C/C++ with
	Clang, add "-Wno-unknown-warning-option" to the options.

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