Suppress warnings on our second compilation (for 64-bit).

Bug: 16031597
Bug: 17052573

Without this patch the 64-bit compilation path will print duplicate warning
diagnostics, since we call the frontend twice (for 32-bit, and then 64-bit).
The simplest fix is to not print warnings for the second compilation. A bug
(17052573) has been filed to track fixing this a better way (actually printing
out all warning diagnostics, but removing duplicates explicitly).

Change-Id: I78ac0ebd2b132713ec0c86c2cf234da2b620eecf
5 files changed