[clang][Tooling] Infer target and mode from argv[0] when using JSONCompilationDatabase

Summary:
Wraps JSON compilation database with a target and mode adding database
wrapper. So that driver can correctly figure out which toolchain to use.

Note that clients that wants to make use of this target discovery mechanism
needs to link in TargetsInfos and initialize them at startup.

Reviewers: ilya-biryukov

Subscribers: mgorny, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63755

llvm-svn: 364386