style: Remove StyleClear::Line

Before this patch, StyleClear::Line indicates that the nsLineBox has break-after
but no float clearance (note we don't allow line break-before with clear:none);
StyleClear::None indicates the line has no break-before nor break-after.

In this patch, I added `mHasForcedLineBreak` bit in nsLineBox to indicate the
line has a break so that StyleClear can serve its original meaning -- the float
clearance. In Part 6, I'll rename more existing methods and variables related to
float clearance.

Now, instead of using StyleClear::None to clear the line break status, the
caller should use ClearForcedLineBreak(); Similar to set
SetInlineLineBreakBeforeAndReset() and SetInlineLineBreakAfter on
nsReflowStatus, SetBreakTypeBefore/SetBreakTypeAfter on nsLineBox always sets
break status with an optional float clearance parameter.

This patch shouldn't change the behavior.

Differential Revision: https://phabricator.services.mozilla.com/D158224

components/style/properties/gecko.mako.rs

@@ -1164,7 +1164,6 @@ fn static_assert() {
         "clear",
         "Left Right None Both",
         gecko_enum_prefix="StyleClear",
-        gecko_inexhaustive=True,
     ) %>
     ${impl_keyword('clear', 'mBreakType', clear_keyword)}
 </%self:impl_trait>