Revert r363633 "[CMake] Fix the value of `config.target_cflags` for non-macOS...

Revert r363633 "[CMake] Fix the value of `config.target_cflags` for non-macOS Apple platforms. Attempt #2."

This caused Chromium's clang package to stop building, see comment on
https://reviews.llvm.org/D61242 for details.

> Summary:
> The main problem here is that `-*-version_min=` was not being passed to
> the compiler when building test cases. This can cause problems when
> testing on devices running older OSs because Clang would previously
> assume the minimum deployment target is the the latest OS in the SDK
> which could be much newer than what the device is running.
>
> Previously the generated value looked like this:
>
> `-arch arm64 -isysroot
> <path_to_xcode>/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS12.1.sdk`
>
> With this change it now looks like:
>
> `-arch arm64 -stdlib=libc++ -miphoneos-version-min=8.0 -isysroot
> <path_to_xcode>/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS12.1.sdk`
>
> This mirrors the setting of `config.target_cflags` on macOS.
>
> This change is made for ASan, LibFuzzer, TSan, and UBSan.
>
> To implement this a new `get_test_cflags_for_apple_platform()` function
> has been added that when given an Apple platform name and architecture
> returns a string containing the C compiler flags to use when building
> tests. This also calls a new helper function `is_valid_apple_platform()`
> that validates Apple platform names.
>
> This is the second attempt at landing the patch. The first attempt (r359305)
> had to be reverted (r359327) due to a buildbot failure. The problem was
> that calling `get_test_cflags_for_apple_platform()` can trigger a CMake
> error if the provided architecture is not supported by the current
> CMake configuration. Previously, this could be triggered by passing
> `-DCOMPILER_RT_ENABLE_IOS=OFF` to CMake. The root cause is that we were
> generating test configurations for a list of architectures without
> checking if the relevant Sanitizer actually supported that architecture.
> We now intersect the list of architectures for an Apple platform
> with `<SANITIZER>_SUPPORTED_ARCH` (where `<SANITIZER>` is a Sanitizer
> name) to iterate through the correct list of architectures.
>
> rdar://problem/50124489
>
> Reviewers: kubamracek, yln, vsk, juliehockett, phosek
>
> Subscribers: mgorny, javed.absar, kristof.beyls, #sanitizers, llvm-commits
>
> Tags: #llvm, #sanitizers
>
> Differential Revision: https://reviews.llvm.org/D61242

llvm-svn: 363779