[cleanup] Make control flow in ParsePrimaryExpression more consistent

[cleanup] Make control flow in ParsePrimaryExpression more consistent

The previous code had a mix of breaks, early returns, and switch/case/if
with fallthrough. Now the pattern is to either return for known errors
or break to the bottom of the switch for unhandled tokens.

Also cleaned up random other stuff in the function: removed unnecessary
local vars, shortened position-fetching calls.

Committed: https://crrev.com/1447f7434410d9f2c21c1e3880ea0607aaf67a63
Cr-Commit-Position: refs/heads/master@{#31843}

[adamk, 2015-11-04 23:27:20]

adamk@chromium.org changed reviewers:
+ littledan@chromium.org

[adamk, 2015-11-04 23:27:20]

This takes care of a TODO I handed caitp in a review awhile back :)

[Dan Ehrenberg, 2015-11-05 21:05:19]

lgtm

[adamk, 2015-11-05 21:11:03]

The CQ bit was checked by adamk@chromium.org

[commit-bot: I haz the power, 2015-11-05 21:11:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1412313009/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1412313009/1

[commit-bot: I haz the power, 2015-11-05 21:13:42]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-05 21:14:15]

Patchset 1 (id:??) landed as
https://crrev.com/1447f7434410d9f2c21c1e3880ea0607aaf67a63
Cr-Commit-Position: refs/heads/master@{#31843}