[Driver] Remove discouraged -gcc-toolchain

[MaskRay]

Summary

Space separated driver options are uncommon but Clang traditionally
did not do a good job. --gcc-toolchain= is the preferred form.

This discourage form appears to be rare, so we can just drop it.

[phosek]

LGTM

clang/include/clang/Driver/Options.td

 def mretpoline_external_thunk : Flag<["-"], "mretpoline-external-thunk">, Group<m_x86_Features_Group>;
 def mno_retpoline_external_thunk : Flag<["-"], "mno-retpoline-external-thunk">, Group<m_x86_Features_Group>;
 def mvzeroupper : Flag<["-"], "mvzeroupper">, Group<m_x86_Features_Group>;
 def mno_vzeroupper : Flag<["-"], "mno-vzeroupper">, Group<m_x86_Features_Group>;
 
 // These are legacy user-facing driver-level option spellings. They are always
 // aliases for options that are spelled using the more common Unix / GNU flag
 // style of double-dash and equals-joined flags.
-def gcc_toolchain_legacy_spelling : Separate<["-"], "gcc-toolchain">, Alias<gcc_toolchain>;
 def target_legacy_spelling : Separate<["-"], "target">, Alias<target>;
 
 // Special internal option to handle -Xlinker --no-demangle.
 def Z_Xlinker__no_demangle : Flag<["-"], "Z-Xlinker-no-demangle">,
     Flags<[Unsupported, NoArgumentUnused]>;
 
 // Special internal option to allow forwarding arbitrary arguments to linker.
 def Zlinker_input : Separate<["-"], "Zlinker-input">,