Utilities/bootstrap: fix ignored --clang-path option

[MaxDesiatov]

Motivation:

Utilities/bootstrap has --clang-path, which currently is parsed, but not propagated anywhere. Ignoring this option caused issues on certain Linux distributions, where the process of bringing up clang is convoluted and bootstrap ended up using GCC, which caused build failures.

Modifications:

args.clang_path value is now passed to CMake as -DCMAKE_C_COMPILER.

Result:

Build failures on Linux in certain configurations are fixed.

[MaxDesiatov]

@swift-ci please smoke test

[MaxDesiatov]

@swift-ci please smoke test

[MaxDesiatov]

@swift-ci please smoke test Linux

[neonichu]

Do we need to do anything to propagate it to the second stage build as well?

[MaxDesiatov]

Do we need to do anything to propagate it to the second stage build as well?

By that, are you referring to "Building SwiftPM (with a freshly built swift-build)" stage? If so, I haven't found a way to instruct swift-build to pick a specific C/C++ compiler so far, other than creating a completely separate destination.json, which I assume is not an option here.

[neonichu]

By that, are you referring to "Building SwiftPM (with a freshly built swift-build)" stage? If so, I haven't found a way to instruct swift-build to pick a specific C/C++ compiler so far, other than creating a completely separate destination.json, which I assume is not an option here.

Yep, that's what I meant. Looking at UserToolchain, it seems as if setting CC in the environment ought to work? If it doesn't in practice, we should fix that.

[MaxDesiatov]

@swift-ci please smoke test

[MaxDesiatov]

@swift-ci please smoke test

[MaxDesiatov]

@tomerd @neonichu this is ready for review