[InputGraph][Gnu] Add LinkerScript support.

This adds LinkerScript support by creating a type Script which is of type
FileNode in the InputGraph. Once the LinkerScript Parser converts the
LinkerScript into a sequence of command, the commands are handled by the
equivalent LinkerScript node for the current Flavor/Target. For ELF, a
ELFGNULdScript gets created which converts the commands to ELF nodes and ELF
control nodes(ELFGroup for handling Group nodes).

Since the Inputfile type has to be determined in the Driver, the Driver needs
to determine the complete path of the file that needs to be processed by the
Linker. Due to this, few tests have been removed since the Driver uses paths
that doesnot exist.
[InputGraph] Fix comment.
Remove the Darwin_Generic_GCC toolchain.

This is currently unused by any test. The code path would still be hit
by clang on ppc, but

* PPC has not been supported on current versions of OS X
* A port of current clang to older OS X on ppc should be using
toolchains::DarwinClang.
remove gcc::Precompile

Clang still has support for running gcc for performing various stages
of a build. Right now it looks like this is used for

* Supporting Fortran in the clang driver
* Running an assembler or linker in systems we don't yet know how to
run them directly.

It looks like the gcc::Precompile is a vestige from the days when we
supported using clang for C and running gcc for c++. This patch removes it
(yes, we have no tests for it).