r110903 introduced a dependency from Frontend to every library that
declared an Action by introducing Action references that previously
resided in the driver in the file ExecuteCompilerInvocation.cpp.
This patch moves ExecuteCompilerInvocation to a new library named
FrontendTool which is intended to bear these dependencies.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@111873 91177308-0d34-0410-b5e6-96231b3b80d8