Tree artifact as input to cc_library fails if it contains object files

[robin-thomas]

Description of the problem / feature request:

If the input to cc_library is a tree artifact, which contains only object files, then bazel build crashes with the below stack trace.

ERROR: Artifact 'bazel-out/k8-fastbuild/bin/<tree_artifact>/<file>.o' expanded from the directory artifact 'bazel-out/k8-fastbuild/bin/<tree_artifact>' is neither header nor source file.
Internal error thrown during build. Printing stack trace: java.lang.IllegalArgumentException: action terminated with unexpected exception: Artifact 'bazel-out/k8-fastbuild/bin/<tree_artifact>/<file>.o' expanded from the directory artifact 'bazel-out/k8-fastbuild/bin/<tree_artifact>' is neither header nor source file.
	at com.google.devtools.build.lib.buildtool.SkyframeBuilder.rethrow(SkyframeBuilder.java:334)
	at com.google.devtools.build.lib.buildtool.SkyframeBuilder.processResult(SkyframeBuilder.java:290)
	at com.google.devtools.build.lib.buildtool.SkyframeBuilder.buildArtifacts(SkyframeBuilder.java:165)
	at com.google.devtools.build.lib.buildtool.ExecutionTool.executeBuild(ExecutionTool.java:341)
	at com.google.devtools.build.lib.buildtool.BuildTool.buildTargets(BuildTool.java:164)
	at com.google.devtools.build.lib.buildtool.BuildTool.processRequest(BuildTool.java:253)
	at com.google.devtools.build.lib.runtime.commands.BuildCommand.exec(BuildCommand.java:83)
	at com.google.devtools.build.lib.runtime.BlazeCommandDispatcher.execExclusively(BlazeCommandDispatcher.java:482)
	at com.google.devtools.build.lib.runtime.BlazeCommandDispatcher.exec(BlazeCommandDispatcher.java:204)
	at com.google.devtools.build.lib.server.GrpcServerImpl.executeCommand(GrpcServerImpl.java:750)
	at com.google.devtools.build.lib.server.GrpcServerImpl.access$1600(GrpcServerImpl.java:103)
	at com.google.devtools.build.lib.server.GrpcServerImpl$2.lambda$run$0(GrpcServerImpl.java:819)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
	at java.base/java.lang.Thread.run(Unknown Source)
Caused by: com.google.devtools.build.lib.actions.ActionTemplate$ActionTemplateExpansionException: Artifact 'bazel-out/k8-fastbuild/bin/<tree_artifact>/<file>.o' expanded from the directory artifact 'bazel-out/k8-fastbuild/bin/<tree_artifact>' is neither header nor source file.
	at com.google.devtools.build.lib.rules.cpp.CppCompileActionTemplate.generateActionForInputArtifacts(CppCompileActionTemplate.java:109)
	at com.google.devtools.build.lib.skyframe.ActionTemplateExpansionFunction.compute(ActionTemplateExpansionFunction.java:79)
	at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:422)
	at com.google.devtools.build.lib.concurrent.AbstractQueueVisitor$WrappedRunnable.run(AbstractQueueVisitor.java:368)
	... 3 more

Feature requests: what underlying problem are you trying to solve with this feature?

Fix tree artifact so that they can have *.o files which can be accepted by cc_library

Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

git clone https://github.com/robin-thomas/bazelify-gmp.git
cd bazelify-gmp
git checkout 18e719924200d486ab101ca35a08f36ba43a68eb
bazel build ...

What operating system are you running Bazel on?

Ubuntu 16.04

What's the output of bazel info release?

release 0.20.0

What's the output of git remote get-url origin ; git rev-parse master ; git rev-parse HEAD ?

git@github.com:robin-thomas/bazelify-gmp.git
18e719924200d486ab101ca35a08f36ba43a68eb
18e719924200d486ab101ca35a08f36ba43a68eb

Have you found anything relevant by searching the web?

This one seems similar: #5092. But the stack trace is different (as that issue looks fixed)

Any other information, logs, or outputs that you want to share?

Seems like the issue is here: https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/rules/cpp/CppCompileActionTemplate.java#L107