When merging views into the enclosing statement
the ORDER BY clause of the view is merged to the
parent's ORDER BY clause.
However when the VIEW is merged into an UNION
branch the ORDER BY should be ignored. 
Use of ORDER BY for individual SELECT statements
implies nothing about the order in which the rows
appear in the final result because UNION by default
produces unordered set of rows.
Fixed by ignoring the ORDER BY clause from the merge
view when expanded in an UNION branch.


mysql-test/r/view.result:
  Bug #27786: test case
mysql-test/t/view.test:
  Bug #27786: test case
sql/sql_lex.h:
  Bug #27786: add a is_union() inlined function
  Returns true if the unit represents an UNION.
sql/sql_view.cc:
  Bug #27786: ignore ORDER BY in mergeable views when in UNION context