[gdb/build] Fix Werror=nonnull-compare build breaker with gcc 12

When building gdb using current gcc trunk, we run into:
...
gdbsupport/gdb_unlinker.h: \
  In constructor 'gdb::unlinker::unlinker(const char*)':
gdbsupport/gdb_assert.h:35:4: error: \
  'nonnull' argument 'filename' compared to NULL [-Werror=nonnull-compare]
35| ((void) ((expr) ? 0 :                                                    \
  | ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
36|          (gdb_assert_fail (#expr, __FILE__, __LINE__, FUNCTION_NAME), 0)))
  |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
gdbsupport/gdb_unlinker.h:38:5: note: in expansion of macro 'gdb_assert'
38|   gdb_assert (filename != NULL);
  |   ^~~~~~~~~~
cc1plus: all warnings being treated as errors
make[1]: *** [compile/compile.o] Error 1
...

The warning triggers in this code:
...
  unlinker (const char *filename) ATTRIBUTE_NONNULL (2)
    : m_filename (filename)
  {
    gdb_assert (filename != NULL);
  }
...

The attribute nonnull (applied here to the filename parameter) has two
effects:
- if the compiler determines that a null pointer is passed in argument
  filename, and the -Wnonnull option is enabled, a warning is issued.
- the compiler may perform optimizations based on the knowledge that
  filename != NULL (unless disabled by the -fno-delete-null-pointer-checks
  option).

The warning Werror=nonnull-compare warns that the compiler may perform the
optimization:
....
    gdb_assert (filename != NULL);
...
->
...
    gdb_assert (true);
...
in which case "unlinker (obfuscated_NULL)" no longer will trigger the assert.
And we want to keep the gdb_assert to detect cases that -Wnonnull doesn't
detect.

We could simply fix this by dropping the attribute, but that means that we no
longer get the -Wnonnull warning.

Fix this by ignoring the nonnull attribute using a function:
...
template<typename T>
const T *volatile
ignore_nonnull (const T *ptr)
{
  return ptr;
}
...
such that we can do:
...
    gdb_assert (ignore_nonnull (filename) != NULL);
...

Build on x86_64-linux using "gcc version 12.0.0 20210727 (experimental) (GCC)"
build from gcc commit 7ffba77d01a.

Reported-By: Jan-Benedict Glaw <jbglaw@lug-owl.de>

gdb/ChangeLog:

2021-07-27  Andrew Burgess  <andrew.burgess@embecosm.com>
	    Tom de Vries  <tdevries@suse.de>

	* gdbsupport/gdb_assert.h (ignore_nonnull): New function.
	* gdb/gdb_regex.c (compiled_regex::compiled_regex): Use ignore_nonnull
	to ignore nonnull attribute on function parameter.
	* gdbsupport/gdb_unlinker.h (unlinker::unlinker): Same.

---
 gdb/gdb_regex.c           | 4 ++--
 gdbsupport/gdb_assert.h   | 6 ++++++
 gdbsupport/gdb_unlinker.h | 2 +-
 3 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/gdb/gdb_regex.c b/gdb/gdb_regex.c
index 17fbfd28ee8..22473f57e26 100644
--- a/gdb/gdb_regex.c
+++ b/gdb/gdb_regex.c
@@ -22,8 +22,8 @@
 compiled_regex::compiled_regex (const char *regex, int cflags,
 				const char *message)
 {
-  gdb_assert (regex != NULL);
-  gdb_assert (message != NULL);
+  gdb_assert (ignore_nonnull (regex) != NULL);
+  gdb_assert (ignore_nonnull (message) != NULL);
 
   int code = regcomp (&m_pattern, regex, cflags);
   if (code != 0)
diff --git a/gdbsupport/gdb_assert.h b/gdbsupport/gdb_assert.h
index 00553a78613..26a9594e7f8 100644
--- a/gdbsupport/gdb_assert.h
+++ b/gdbsupport/gdb_assert.h
@@ -58,4 +58,10 @@
   internal_error (__FILE__, __LINE__, _(message))
 #endif
 
+template<typename T>
+const T *volatile
+ignore_nonnull (const T *ptr)
+{
+  return ptr;
+}
 #endif /* COMMON_GDB_ASSERT_H */
diff --git a/gdbsupport/gdb_unlinker.h b/gdbsupport/gdb_unlinker.h
index bda6fe7ab54..4aa4f3fc3e9 100644
--- a/gdbsupport/gdb_unlinker.h
+++ b/gdbsupport/gdb_unlinker.h
@@ -35,7 +35,7 @@ class unlinker
   unlinker (const char *filename) ATTRIBUTE_NONNULL (2)
     : m_filename (filename)
   {
-    gdb_assert (filename != NULL);
+    gdb_assert (ignore_nonnull (filename) != NULL);
   }
 
   ~unlinker ()