cmIDEOptions: Factor FlagMap update out to separate method

This will allow it to be re-used in other code paths.
VS: Support mapping flags with values following separately (#14858)

Add a "UserFollowing" special flag table entry indicator to say that a
flag expects a value in a following argument.  Teach cmIDEOptions to
handle such flags.
VS: Fix /analyze:log flag mapping (#14858)

Fix the VS 11 and VS 12 flag table entries for this flag.  It requires
a value in the following argument.  Also drop the general "/analyze:"
flag table entry so that such flags will be passed through as plain
additional options.  This is necessary because some such options have
following values and some do not but not all have .vcxproj elements
to hold the values.
Merge topic 'vs-flag-with-following-value' into next

e8633e66 VS: Fix /analyze:log flag mapping (#14858)
650199e7 VS: Support mapping flags with values following separately (#14858)
f2a3dd9d cmIDEOptions: Factor FlagMap update out to separate method