[Tooling] A new framework for executing clang frontend actions.

Summary:
This defines a `clang::tooling::ToolExecutor` interface that can be extended to support different execution plans including standalone execution on a given set of TUs or parallel execution on all TUs in a codebase.

In order to enable multiprocessing execution, tool actions are expected to output result into a `ToolResults` interface provided by executors. The `ToolResults` interface abstracts how results are stored e.g. in-memory for standalone executions or on-disk for large-scale execution.

New executors can be registered as `ToolExecutorPlugin`s via the `ToolExecutorPluginRegistry`. CLI tools can use `createExecutorFromCommandLineArgs` to create a specific registered executor according to the command-line arguments.

This patch also implements `StandaloneToolExecutor` which has the same behavior as the current `ClangTool` interface, i.e. execute frontend actions on a given set of TUs. At this point, it's simply a wrapper around `ClangTool` at this point.

This is still experimental but expected to replace the existing `ClangTool` interface so that specific tools would not need to worry about execution.

Reviewers: klimek, arphaman, hokein, sammccall

Reviewed By: klimek

Subscribers: cfe-commits, djasper, mgorny, omtcyfz

Differential Revision: https://reviews.llvm.org/D34272

llvm-svn: 316653