sqlite: Re-enable ENABLE_SQLITE_API_ARMOR outside fuzzers.

https://crrev.com/c/1315959 and https://crrev.com/c/1341921 made it
possible to use SQLITE_DEBUG for SQLite fuzzers, which has uncovered
bugs. Unfortunately, the CLs unintentionally removed
ENABLE_SQLITE_API_ARMOR from non-fuzzing builds, reducing our ability to
catch API misuse.

This CL re-instates ENABLE_SQLITE_API_ARMOR for non-fuzzing builds, for
the reasons described above. It also removes
-Wno-implicit-function-declaration in return for a less intrusive
workaround in sqlite3_shim.c.

Bug: 900910
Change-Id: I6327fdcee173c384da0b3b62c1414b7b6126473f
Reviewed-on: https://chromium-review.googlesource.com/c/1345649
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610362}