Previously, completing inside non single expression body might cause a
crash if you had done any completion with single expression in the same
body.
e.g.
func test() {
test(#^HERE^#)
}
after that:
func test() {
test(arg)
if #^HERE^#
}
That was because the `hasSingleExpressionBody` wasn't cleared when
reusing the function for subsequent completions.
This patch just clears it whenever a new parsed body is set to a
function.
rdar://75358153
