Use multiarch layout with multilibs for libc++ headers

[petrhosek]

For the following multilib.yaml file:

MultilibVersion: 1.0

Variants:
- Dir: testdir1
  Flags: [--target=thumbv7m-none-unknown-eabi]

- Dir: testdir2
  Flags: [--target=thumbv7em-none-unknown-eabi]

Mappings:
- Match: --target=thumbv7em-none-unknown-eabi
  Flags: [--target=thumbv7m-none-unknown-eabi]

We'll end up with the following include directories:

-internal-isystem [[SYSROOT]]/bin/../lib/clang-runtimes/testdir1/include/c++/v1"
-internal-isystem [[SYSROOT]]/bin/../lib/clang-runtimes/testdir2/include/c++/v1"

This is suboptimal. libc++ headers are designed to be target agnostic with the exception of __config_site to avoid having to distribute multiple copies of headers, one for each target or multilib.

Ideally, what we should end up with is something like this:

-internal-isystem [[SYSROOT]]/bin/../include/thumbv7m-none-unknown-eabi/testdir1/include/c++/v1
-internal-isystem [[SYSROOT]]/bin/../include/thumbv7m-none-unknown-eabi/testdir2/include/c++/v1
-internal-isystem [[SYSROOT]]/bin/../include/c++/v1

Where [[SYSROOT]]/bin/../include/thumbv7m-none-unknown-eabi/testdir1/include/c++/v1 and [[SYSROOT]]/bin/../include/thumbv7m-none-unknown-eabi/testdir2/include/c++/v1 should only contain __config_site.

[llvmbot]

@llvm/issue-subscribers-clang-driver

Author: Petr Hosek (petrhosek)

For the following `multilib.yaml` file: ``` --- MultilibVersion: 1.0

Variants:

• Dir: testdir1
  Flags: [--target=thumbv7m-none-unknown-eabi]

• Dir: testdir2
  Flags: [--target=thumbv7em-none-unknown-eabi]

Mappings:

• Match: --target=thumbv7em-none-unknown-eabi
  Flags: [--target=thumbv7m-none-unknown-eabi]

We'll end up with the following include directories:

-internal-isystem [[SYSROOT]]/bin/../lib/clang-runtimes/testdir1/include/c++/v1"
-internal-isystem [[SYSROOT]]/bin/../lib/clang-runtimes/testdir2/include/c++/v1"

This is suboptimal. libc++ headers are designed to be target agnostic with the exception of `__config_site` to avoid having to distribute multiple copies of headers, one for each target or multilib.

Ideally, what we should end up with is something like this:

-internal-isystem [[SYSROOT]]/bin/../include/thumbv7m-none-unknown-eabi/testdir1/include/c++/v1
-internal-isystem [[SYSROOT]]/bin/../include/thumbv7m-none-unknown-eabi/testdir2/include/c++/v1
-internal-isystem [[SYSROOT]]/bin/../include/c++/v1

Where `[[SYSROOT]]/bin/../include/thumbv7m-none-unknown-eabi/testdir1/include/c++/v1` and `[[SYSROOT]]/bin/../include/thumbv7m-none-unknown-eabi/testdir2/include/c++/v1` should only contain `__config_site`.
</details>