scons,debug: Implement the "All" flag in C++ and not scons.

Create an AllFlagsFlag class which inherits from the CompoundFlag class.
This class is a singleton, and the SimpleFlags install themselves in it
instead of having SCons collect them.

The allFlagsVersion global variable was supposed to be for debugging
according to a comment, but was actually an important part of the "All"
flags inner workings. It was not exposed in the header, but was
redefined/pulled through in src/python/pybind11/debug.cc. The
AllFlagsFlag class now tracks that value, and it can be accessed without
reaching behind the curtain.

This also somewhat decentralizes the debug flag building process in
SCons. The debug/flags.cc still includes all flags at once which
centralizes them, but at least now the "All" flag won't also.

Change-Id: I8430e0fe9022846aade028fb46c80777169a2007
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48370
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Nathanael Premillieu <nathanael.premillieu@huawei.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

src/SConscript

@@ -594,13 +594,17 @@ Export('GTest')
 #
 debug_flags = {}
 def DebugFlag(name, desc=None, fmt=False):
+    if name == "All":
+        raise AttributeError('The "All" flag name is reserved')
     if name in debug_flags:
-        raise AttributeError("Flag {} already specified".format(name))
+        raise AttributeError('Flag {} already specified'.format(name))
     debug_flags[name] = (name, (), desc, fmt)
 
 def CompoundFlag(name, flags, desc=None):
+    if name == "All":
+        raise AttributeError('The "All" flag name is reserved')
     if name in debug_flags:
-        raise AttributeError("Flag {} already specified".format(name))
+        raise AttributeError('Flag {} already specified'.format(name))
 
     compound = tuple(flags)
     debug_flags[name] = (name, compound, desc, False)
@@ -608,19 +612,6 @@ def CompoundFlag(name, flags, desc=None):
 def DebugFormatFlag(name, desc=None):
     DebugFlag(name, desc, True)
 
-# Create a compound debug flag that encapsulates all flags: "All". This
-# flag should not be used within C++ code - it is a compound meta flag
-def _createAllDebugFlag():
-    simple_flags = []
-    for name,flag in sorted(debug_flags.items()):
-        n, compound, desc, fmt = flag
-        assert n == name
-        if not compound and not fmt:
-            simple_flags.append(n)
-
-    CompoundFlag("All", simple_flags,
-        "Controls all debug flags. It should not be used within C++ code.")
-
 Export('DebugFlag')
 Export('CompoundFlag')
 Export('DebugFormatFlag')
@@ -1156,7 +1147,6 @@ namespace debug
     code.write(str(target[0]))
 
 # Generate the files for the debug and debug-format flags
-_createAllDebugFlag()
 for name,flag in sorted(debug_flags.items()):
     n, compound, desc, fmt = flag
     assert n == name