Add a new tag to NDK symbol files: future.

Symbols that have been added to a library but should not be exposed
in any of the current NDK API levels should be tagged with "future".
These will be suppressed from the NDK libraries.

Once all this is in better shape we'll have `sdk: "current"`. Symbols
tagged with "future" will be available for that.

Note that this tag can be applied directly to a version. Aside from
being more ergonomic than tagging an entire section, this also solved
the problem of gen_stub_libs.py emitting an empty global section
(which is not valid syntax) in the case where every symbol is
"future". Tag the version instead and it will be omitted.

Test: `make ndk` with libc/libm migration patches.
Change-Id: I41f6e4939c406f695ab5725f360ec6554ad8ab31

cc/gen_stub_libs.py

@@ -65,6 +65,12 @@ class Stack(object):
         return self.stack[-1]
 
 
+def get_tags(line):
+    """Returns a list of all tags on this line."""
+    _, _, all_tags = line.strip().partition('#')
+    return re.split(r'\s+', all_tags)
+
+
 def version_is_private(version):
     """Returns True if the version name should be treated as private."""
     return version.endswith('_PRIVATE') or version.endswith('_PLATFORM')
@@ -78,7 +84,8 @@ def enter_version(scope, line, version_file):
     # Entering a new version block. By convention symbols with versions ending
     # with "_PRIVATE" or "_PLATFORM" are not included in the NDK.
     version_name = line.split('{')[0].strip()
-    if version_is_private(version_name):
+    tags = get_tags(line)
+    if version_is_private(version_name) or 'future' in tags:
         scope.push(Scope.Private)
     else:
         scope.push(Scope.Global)  # By default symbols are visible.
@@ -166,6 +173,11 @@ def symbol_in_version(tags, arch, version):
         elif tag.startswith('introduced-' + arch + '='):
             introduced_tag = tag
             arch_specific = True
+        elif tag == 'future':
+            # This symbol is not in any released API level.
+            # TODO(danalbert): These need to be emitted for version == current.
+            # That's not a construct we have yet, so just skip it for now.
+            return False
 
     if introduced_tag is None:
         # We found no "introduced" tags, so the symbol has always been
@@ -194,8 +206,7 @@ def handle_global_scope(scope, line, src_file, version_file, arch, api):
     # Line is now in the format "<symbol-name>; # tags"
     # Tags are whitespace separated.
     symbol_name, _, rest = line.strip().partition(';')
-    _, _, all_tags = rest.partition('#')
-    tags = re.split(r'\s+', all_tags)
+    tags = get_tags(line)
 
     if not symbol_in_arch(tags, arch):
         return