Fix exprTypmod to recognize length-coercion function expressions,

Fix exprTypmod to recognize length-coercion function expressions,
such as bpchar(char_expression, N), and pull out the attrtypmod that
the function is coercing to.  This allows correct deduction of the
column type in examples such as
CREATE VIEW v AS SELECT f1::char(8) FROM tbl;
Formerly we labeled v's column as char-of-unknown-length not char(8).
Also, this change causes the parser not to insert a redundant length
coercion function if the user has explicitly casted an INSERT or UPDATE
expression to the right length.