Remove -i command line option, add -imultilib

[erichkeane]

Summary

I discovered that '-i' is a command line option for the driver,
however it actually does not do anything and is not supported by any
other compiler. In fact, it is completely undocumented for Clang.

I found a couple of instances of people confusing it with one of
the variety of other command line options that control the driver.
Because of this, we should delete this option so that it is clear
that it isn't valid.

HOWEVER, I found that GCC DOES support -imultilib, which the -i
was hiding our lack of support for. We currently only use imultilib
for the purpose of forwarding to gfortran (in a specific test written
by chandlerc for this purpose).

imultilib is a rarely used (if ever?) feature that I could find no
references to on the internet, and in fact, my company's massive test
suite has zero references to it ever being used.

SO, this patch removes the -i option so that we will now give an error
on its usage (so that it won't be confused with -I), and replaces it with
-imultilib, which is now specified as a gfortran_group option.

[aaron.ballman]

[erichkeane]

Talked to Aaron and came up with a way of covering with a testcase.

[aaron.ballman]

LGTM!

cfe/trunk/include/clang/Driver/Options.td

 def iwithprefix : JoinedOrSeparate<["-"], "iwithprefix">, Group<clang_i_Group>, Flags<[CC1Option]>,
   HelpText<"Set directory to SYSTEM include search path with prefix">, MetaVarName<"<dir>">;
 def iwithsysroot : JoinedOrSeparate<["-"], "iwithsysroot">, Group<clang_i_Group>,
   HelpText<"Add directory to SYSTEM include search path, "
            "absolute paths are relative to -isysroot">, MetaVarName<"<directory>">,
   Flags<[CC1Option]>;
 def ivfsoverlay : JoinedOrSeparate<["-"], "ivfsoverlay">, Group<clang_i_Group>, Flags<[CC1Option]>,
   HelpText<"Overlay the virtual filesystem described by file over the real file system">;
-def i : Joined<["-"], "i">, Group<i_Group>;
+def imultilib : Separate<["-"], "imultilib">, Group<gfortran_Group>;
 def keep__private__externs : Flag<["-"], "keep_private_externs">;
 def l : JoinedOrSeparate<["-"], "l">, Flags<[LinkerInput, RenderJoined]>,
         Group<Link_Group>;
 def lazy__framework : Separate<["-"], "lazy_framework">, Flags<[LinkerInput]>;
 def lazy__library : Separate<["-"], "lazy_library">, Flags<[LinkerInput]>;
 def mlittle_endian : Flag<["-"], "mlittle-endian">, Flags<[DriverOption]>;
 def EL : Flag<["-"], "EL">, Alias<mlittle_endian>;
 def mbig_endian : Flag<["-"], "mbig-endian">, Flags<[DriverOption]>;

cfe/trunk/test/Driver/unknown-arg.c

-// RUN: not %clang %s -cake-is-lie -%0 -%d -HHHH -munknown-to-clang-option -print-stats -funknown-to-clang-option -### 2>&1 | \
+// RUN: not %clang %s -cake-is-lie -%0 -%d -HHHH -munknown-to-clang-option -print-stats -funknown-to-clang-option -ifoo -imultilib dir -### 2>&1 | \
 // RUN: FileCheck %s
+// RUN: %clang %s -imultilib dir -### 2>&1 | \
+// RUN: FileCheck %s --check-prefix=MULTILIB
 // RUN: not %clang %s -stdlibs=foo -hell -version -### 2>&1 | \
 // RUN: FileCheck %s --check-prefix=DID-YOU-MEAN
 // RUN: %clang_cl -cake-is-lie -%0 -%d -HHHH -munknown-to-clang-option -print-stats -funknown-to-clang-option -### -c -- %s 2>&1 | \
 // RUN: FileCheck %s --check-prefix=CL
 // RUN: %clang_cl -Brepo -### -- %s 2>&1 | \
 // RUN: FileCheck %s --check-prefix=CL-DID-YOU-MEAN
 // RUN: not %clang_cl -cake-is-lie -%0 -%d -HHHH -munknown-to-clang-option -print-stats -funknown-to-clang-option -c -Werror=unknown-argument -### -- %s 2>&1 | \
 // RUN: FileCheck %s --check-prefix=CL-ERROR
 // RUN: not %clang_cl -helo -Werror=unknown-argument -### -- %s 2>&1 | \
 // RUN: FileCheck %s --check-prefix=CL-ERROR-DID-YOU-MEAN
 // RUN: %clang_cl -cake-is-lie -%0 -%d -HHHH -munknown-to-clang-option -print-stats -funknown-to-clang-option -c -Wno-unknown-argument -### -- %s 2>&1 | \
 // RUN: FileCheck %s --check-prefix=SILENT
 // RUN: not %clang -cc1as -hell --version -debug-info-macros 2>&1 | \
 // RUN: FileCheck %s --check-prefix=CC1AS-DID-YOU-MEAN
 // RUN: not %clang -cc1asphalt -help 2>&1 | \
 // RUN: FileCheck %s --check-prefix=UNKNOWN-INTEGRATED
 
 // CHECK: error: unknown argument: '-cake-is-lie'
 // CHECK: error: unknown argument: '-%0'
 // CHECK: error: unknown argument: '-%d'
 // CHECK: error: unknown argument: '-HHHH'
 // CHECK: error: unknown argument: '-munknown-to-clang-option'
 // CHECK: error: unknown argument: '-print-stats'
 // CHECK: error: unknown argument: '-funknown-to-clang-option'
+// CHECK: error: unknown argument: '-ifoo'
+// MULTILIB: warning: argument unused during compilation: '-imultilib dir'
 // DID-YOU-MEAN: error: unknown argument '-stdlibs=foo', did you mean '-stdlib=foo'?
 // DID-YOU-MEAN: error: unknown argument '-hell', did you mean '-help'?
 // DID-YOU-MEAN: error: unknown argument '-version', did you mean '--version'?
 // CL: warning: unknown argument ignored in clang-cl: '-cake-is-lie'
 // CL: warning: unknown argument ignored in clang-cl: '-%0'
 // CL: warning: unknown argument ignored in clang-cl: '-%d'
 // CL: warning: unknown argument ignored in clang-cl: '-HHHH'
 // CL: warning: unknown argument ignored in clang-cl: '-munknown-to-clang-option'