[Sema] More changes to fix Objective-C fallout from r249995.

[Sema] More changes to fix Objective-C fallout from r249995.

This is a follow-up to PR26085. That was fixed in r257710 but the testcase
there was incomplete. There is a related issue where the overload resolution
for Objective-C incorrectly picks a method that is not valid without a
bridge cast. The call to Sema::CheckSingleAssignmentConstraints that was
added to SemaOverload.cpp's IsStandardConversion() function does not catch
that case and reports that the method is Compatible even when it is not.

The root cause here is that various Objective-C-related functions in Sema
do not consistently return a value to indicate whether there was an error.
This was fine in the past because they would report diagnostics when needed,
but r257710 changed them to suppress reporting diagnostics when checking
during overload resolution.

This patch adds a new ACR_error result to the ARCConversionResult enum and
updates Sema::CheckObjCARCConversion to return that value when there is an
error. Most of the calls to that function do not check the return value,
so adding this new result does not affect them. The one exception is in
SemaCast.cpp where it specifically checks for ACR_unbridged, so that is
also OK. The call in Sema::CheckSingleAssignmentConstraints can then check
for an ACR_okay result and identify assignments as Incompatible. To
preserve the existing behavior, it only changes the return value to
Incompatible when the new Diagnose argument (from r257710) is false.

Similarly, the CheckObjCBridgeRelatedConversions and
ConversionToObjCStringLiteralCheck need to identify when an assignment is
Incompatible. Those functions already return appropriate values but they
need some fixes related to the new Diagnose argument.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260787 91177308-0d34-0410-b5e6-96231b3b80d8