firebase-ios-sdk

Update project and fix warnings (#2531)

* Update Firestore project to Xcode 10.1 defaults

* Fix NSUInteger in format strings warning

* Fix implicit conversion warning in FSTLevelDB

* Fix BOOL/bool mismatch

* Fix erroneous comparison

This failure shows up on 32-bit targets. I have no idea why this
succeeds on 64-bit targets.

* Fix incorrect type inference with explicit cast

clang is inferring the type of the key in these maps is `NSString *` and
then complaining that `FIRFieldValue *` doesn't match that type, despite
the underlying method taking `NSDictionary<id, id>`. Explicitly casting
to `id` prevents the warning.

* Remove reliance on representation in FSTFieldValue tests

On *some* platforms [NSNumber numberWithChar:1] ends up with a "c"
(signed char) objcType, but apparently on 32-bit platforms this ends up
as "i" (signed int). Rather than relying on this working properly just
test that [NSNumber numberWithBool:] does the right thing.

* Fix field_value_test on 32-bit targets

When sizeof(void*) is 4 the test failed :-(.

* Rephrase testWritesWithLargeNumbersFail in terms of fixed size types

The prior definition only worked on LP64. Switching to LLONG_MAX would
have also fixed this, but Firestore's data model is independent of C's
data model, so rewriting this in terms of fixed size types more closely
models what's actually an invalid value.

* Fix implicit retain self warnings