[llvm] r199886 - Prevent repetitive warnings for unrecognized processors and features

[Artyom Skrobov]

> These are backend diagnostics only, right? I.e., they get issued by MC
> when using developer tools like llvm-mc, llc, et. al.. They're
> certainly not diagnostics we would ever expect to see from, e.g.,
> clang or other user-level front-end. As such, I don't quite follow
> why we should care about filtering duplicates at all. I don't oppose
> uniquing them or anything, I just don't personally see it as a
> priority. YMMV.

Yes, that's how it's supposed to be; but occasionally, these diagnostics
"leak out" of the backend: one particular case that we're aware of, is

$ clang -target arm -mcpu=cortex-r4 -c test.c
clang-3.5: warning: unknown platform, assuming -mfloat-abi=soft
clang-3.5: warning: unknown platform, assuming -mfloat-abi=soft
'cortex-r4' is not a recognized processor for this target (ignoring
processor)
'cortex-r4' is not a recognized processor for this target (ignoring
processor)
'cortex-r4' is not a recognized processor for this target (ignoring
processor)
'cortex-r4' is not a recognized processor for this target (ignoring
processor)
'cortex-r4' is not a recognized processor for this target (ignoring
processor)
'cortex-r4' is not a recognized processor for this target (ignoring
processor)