[clang] Reland "[clang][modules-driver] Add support for C++ named modules and import std" (2nd attempt) (PR #194475)

[Rainer Orth via cfe-commits]

rorth wrote:

This PR broke the [Solaris/sparcv9 buildbot](https://lab.llvm.org/buildbot/#/builders/13/builds/12932) because the `Clang::modules-driver-import-std.cpp` test unconditionally uses `--target=x86_64-linux-gnu` which is only available with `-DLLVM_TARGETS_TO_BUILD=all` or on native x86 targets.

https://github.com/llvm/llvm-project/pull/194475