Driver: emit a diagnostic if clang++ is not found

[compnerd]

Rather than aborting due to an assertion failure, emit a diagnostic.
This is much safer and generally easier to understand why the command
failed. It solves the problem of running swiftc from the build without
the path being set such that the clang++ driver is found by the swift
driver.

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[compnerd]

@swift-ci please test

[compnerd]

@swift-ci please test Windows platform

[compnerd]

CC: @jrose-apple

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 25dfd00c6ec976e0c5a720381f12d9b73e9fbb15

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 25dfd00c6ec976e0c5a720381f12d9b73e9fbb15

[jrose-apple]

Toolchains aren't supposed to diagnose during argument construction right now. I realize that's not enforced, but we've been doing that so far. All the other tools don't assert; they just build a command line with the missing command's name so that trying to execute them gives a "not found" error.

[compnerd]

Alrighty, I'll just change the behavior of the toolchain here then, thanks @jrose-apple!

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 25dfd00c6ec976e0c5a720381f12d9b73e9fbb15

[compnerd]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - a6c377a9c66a0b380357a90b043bda1b1f186930

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - a6c377a9c66a0b380357a90b043bda1b1f186930

[compnerd]

Apparently, setting PATH to empty is a bad idea ...

[compnerd]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 47eb0f17b21615591a361bff4e9dce32beb8b8d0

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 47eb0f17b21615591a361bff4e9dce32beb8b8d0

[compnerd]

@jrose-apple - sigh I don't really know how to test this. We need to ensure that SWIFT_BIN_INT_OUTDIR, /bin, and /usr/bin are not in PATH when the driver is run, and seems that emptying PATH is not exactly something which works well.

[jrose-apple]

Well, it doesn't belong in "subcommands.swift". :-) That's things like swift build and swift package.

I don't know if I have a good answer here either. The best I can think of is setting up an artifical bin/ folder with the things you need to get to the point of generating jobs. The next best is adding a driver flag that lets you pretend all exec lookups fail or something.

[compnerd]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 47eb0f17b21615591a361bff4e9dce32beb8b8d0

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 47eb0f17b21615591a361bff4e9dce32beb8b8d0

[compnerd]

I just realized that this is easy to test - because we no longer do the search for clang++, as long as -tools-directory is not specified, then we should get an explicit "clang++" rather than /usr/bin/clang.

[compnerd]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 3a34b2a7bb8b3e1f4cf11d48a4cd0848c8fa3f7b

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 3a34b2a7bb8b3e1f4cf11d48a4cd0848c8fa3f7b

[compnerd]

@swift-ci please test macOS platform

[davezarzycki]

This broke unified builds on Linux (Fedora 30, x86-64). Was the following expected?

/home/dave/s/u/swift/test/Driver/windows-link-job.swift:2:11: error: CHECK: expected string not found in input
// CHECK: {{^}}clang++
          ^
<stdin>:1:1: note: scanning from here
/home/dave/b/u/t/bin/swift -frontend -c -primary-file SOURCE_DIR/test/Driver/windows-link-job.swift -target x86_64-unknown-windows-msvc -disable-objc-interop -autolink-library oldnames -autolink-library msvcrt -Xcc -D_MT -Xcc -D_DLL -parse-as-library -module-name link -o /tmp/lit_tmp_SROuQs/windows-link-job-138cf2.o
^
<stdin>:2:19: note: possible intended match here
/home/dave/b/u/t/bin/clang++ -shared -target x86_64-unknown-windows-msvc -nostartfiles -L /home/dave/b/u/t/lib/swift/windows/x86_64 /home/dave/b/u/t/lib/swift/windows/x86_64/swiftrt.obj /tmp/lit_tmp_SROuQs/windows-link-job-138cf2.o -o link.dll
                  ^

[davezarzycki]

@compnerd – FYI: #25972