Move ExecuteCompilerInvocation to a new library FrontendTool

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.

llvm-svn: 111873