Fix variable usage tracking in erl_lint

When analyzing complex expressions (i.e. comprehensions, cases, tries,
ifs and receives), erl_lint does not forget about old unused variables
when returning the updated variable table. This causes a bug where old
unused variables are not recorded as such:

t(X, Y) ->
  #r{a=[ K || K <- Y ],b=[ K || K <- Y ]}.

As erl_lint uses vtmerge_pat/2 to merge the results of the analysis of
the two list comprehensions, X is marked as used and the warning is not
emitted.

The function vtmerge_pat/2 is used instead of the similar vtmerge/2
which does not mark multiple occurrences of a variable as usage to
handle cases like the following one:

t(X, Y) ->
  #r{a=A=X,b=A=Y}.

Other simpler expressions like conses, tuples and external fun
references do not correctly follow this behaviour, e.g. A is not marked
as used in the following code:

t(X, Y) ->
  {A=X,A=Y}.

This commit fixes both issues and makes erl_lint not return old unused
variables in updated tables and makes all compound expressions use
vtmerge_pat/2.

Reported-by: Anders Ramsell
Fix unsafe variable tracking in try expressions

Variables used in the body of a try expression were marked as unsafe
*and* used, which makes no sense as an unsafe variable can't be used.

Function vtsubtract/2 is used to forget usage of such unsafe variables.

Reported-by: Paul Davis
Fix variable usage tracking in some record errors

When reporting a field redefinition in a record, erl_lint can forget
about some old unused variables.

f() -> X = 1, #r{a=foo,a=bar,a=qux}.
Change encoding of troublesome notes.xml files to utf-8

Most notes.xml files will be updated in every release and cause
the kind of the problems described in the previous commit.
Merge branch 'bjorn/xml-encoding-fix/OTP-11310' into maint

* bjorn/xml-encoding-fix/OTP-11310:
  Change encoding of troublesome notes.xml files to utf-8
  Convert some notes.xml files from latin-1 to utf-8