Adds a tooling library.

Adds a tooling library.

Provides an API to run clang tools (FrontendActions) as standalone tools,
or repeatedly in-memory in a process. This is useful for unit-testing,
map-reduce style applications, source transformation daemons or command line
tools.

The ability to run over multiple translation units with different command
line arguments enables building up refactoring tools that need to apply
transformations across translation unit boundaries.

See tools/clang-check/ClangCheck.cpp for an example.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@154008 91177308-0d34-0410-b5e6-96231b3b80d8