clang/apple: Infer simulator env from -mios-simulator-version-min= flag

Before this patch, open-source clang would consider
`-target x86_64-apple-darwin -mios-simulator-version-min=11.0` as
targeting the iOS simulator, due to the mios flag informing it
that we want to target iOS, and logic in the driver then realizing
that x86 iOS builds must be the simulator.

However, for `-target arm64-apple-darwin -mios-simulator-version-min=11.0`
that didn't work and clang thought that it's building for actual iOS,
and not for the simulator.

Due to this, building compiler-rt for arm64 iossim would lead to
all .o files in RTSanitizerCommonSymbolizer.iossim.dir being built
for iOS instead of for iOS simulator, and clang would ask ld64 to
link for iOS, but using the iPhoneSimulator sysroot. This would then
lead to many warnings from ld64 looking like:

    ld: warning: building for iOS, but linking in .tbd file
        (.../iPhoneSimulator.sdk/usr/lib/libc++abi.tbd) built for iOS Simulator

Worse, with ld64.lld, this diagnostic is currently an error instead
of a warning.

This patch makes it so that the presence of -mios-simulator-version-min=
now informs clang that we're building for simulator. That way, all the
.o files are built for simulator, the linker is informed that we're
building for simulator, and everything Just Works.

(Xcode's clang already behaves like this, so this makes open-source clang
match Xcode clang.)

We can now likely remove the hack to treat non-mac darwin x86 as
simulator, but doing that feels slightly risky, so I'm leaving that
for a follow-up patch.

(This patch is made necessary by the existence of arm64 macs.)

Differential Revision: https://reviews.llvm.org/D132258

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