[driver] Search for compatible Android runtime directories

Android triples include a version number, which makes direct triple
comparisons for per-target runtime directory searching not always work.

[driver] Search for compatible Android runtime directories

Android triples include a version number, which makes direct triple
comparisons for per-target runtime directory searching not always work.
Instead, look for the triple with the highest compatible version number
and use that per-target runtime directory instead. This maintains the
existing fallback to a triple without any version number, but I'm hoping
we can remove that in the future. https://discourse.llvm.org/t/62717
discusses this further.

The one remaining triple mismatch after this is that Android armv7
triples usually have an environment of `androideabi`, which Clang
normalizes to `android`. If you use the `androideabi` triple when
building the runtimes with a per-target runtimes dir, the directory will
get created with `androideabi` in its name, but Clang's triple search
uses the normalized triple and will look for an `android` directory
instead. https://reviews.llvm.org/D140925 will fix that by normalizing
triples when creating the per-target runtimes directories as well.

Reviewed By: phosek, pirama

Differential Revision: https://reviews.llvm.org/D158476

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[driver] Perform fallback target searches for stdlib

Searching for target-specific standard library header and library paths
should perform fallback searches for targets, the same way searching for

[driver] Perform fallback target searches for stdlib

Searching for target-specific standard library header and library paths
should perform fallback searches for targets, the same way searching for
the runtime libraries does. It's important for the header and library
searches to be consistent, otherwise we could end up using mismatched
headers and libraries. (See also https://reviews.llvm.org/D146664.)

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D159293

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[driver] Conditionally include installed libc++ headers for Android

https://reviews.llvm.org/D71154 prevented Clang from search for libc++
headers installed alongside the driver when targeting Andro

[driver] Conditionally include installed libc++ headers for Android

https://reviews.llvm.org/D71154 prevented Clang from search for libc++
headers installed alongside the driver when targeting Android. The
motivation was the NDK's use of a different libc++ inline namespace
(`__ndk1` instead of the standard `__1`), which made regular libc++
headers incompatible with the NDK's libc++ library.

Since then, libc++ has gained the ability to install its `__config_site`
header (which controls the inline namespace, among other things) to a
per-target include directory, which enables per-target customizations.
If this directory is present, the user has expressly built libc++ for
Android, and we should use those headers.

The motivation is that, with the current setup, if a user builds their
own libc++ for Android, they'll use the library they built themselves
but the NDK's headers instead of their own, which is surprising at best
and can cause all sorts of problems (e.g. if you built your own libc++
with a different ABI configuration). It's important to match the headers
and libraries in that scenario, and checking for an Android per-target
include directory lets us do so without regressing the original scenario
which https://reviews.llvm.org/D71154 was addressing.

While I'm here, switch to using sys::path::append instead of slashes
directly, to get system path separators on Windows, which is consistent
with how library paths are constructed (and that consistency will be
important in a follow-up, where we use a common search function for the
include and library path construction).

(As an aside, one of the motivations for https://reviews.llvm.org/D71154
was to support targeting both Android and Apple platforms, which
expected libc++ headers to be provided by the toolcain at the time.
Apple has since switched to including libc++ headers in the platform SDK
instead of in the toolchain, so that specific motivation no longer
applies either.)

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D159292

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