[Clang][Docs] Add help test to `-march` and `-mcpu` to suggest `-mcpu=help`

[jhuber6]

Summary

Currently there is no documentation for these flags, users might find it
confusing to know which values are permitted. This change simply adds
documentation to suggest using -mcpu=help to list the availible target
names.

[aaron.ballman]

LGTM, though it's a bit jarring that -march suggests using -mcpu=help (should we ensure that -march=help works on all targets instead?)

[jhuber6]

In D151098#4360750, @aaron.ballman wrote:

LGTM, though it's a bit jarring that -march suggests using -mcpu=help (should we ensure that -march=help works on all targets instead?)

Using -march=help works inadvertently because it will list the targets when given an invalid one. We could probably map the -mcpu=help handling to -march=help but I'm not entirely sure what the semantics should be there. The distinction between -march and -mcpu is still not entirely clear to me, it's just a matter of whether or not the target decided to choose one or the other.

[aaron.ballman]

In D151098#4360758, @jhuber6 wrote:

In D151098#4360750, @aaron.ballman wrote:

LGTM, though it's a bit jarring that -march suggests using -mcpu=help (should we ensure that -march=help works on all targets instead?)

Using -march=help works inadvertently because it will list the targets when given an invalid one. We could probably map the -mcpu=help handling to -march=help but I'm not entirely sure what the semantics should be there. The distinction between -march and -mcpu is still not entirely clear to me, it's just a matter of whether or not the target decided to choose one or the other.

Oh, when I was trying it out, -march=help was not giving any useful information on some targets: https://godbolt.org/z/js4a64aTK -- FWIW, it's also unclear to me why and how these options differ.

clang/include/clang/Driver/Options.td

 def mno_pure_code : Flag<["-"], "mno-pure-code">, Alias<mno_execute_only>;
 def mtvos_version_min_EQ : Joined<["-"], "mtvos-version-min=">, Group<m_Group>;
 def mappletvos_version_min_EQ : Joined<["-"], "mappletvos-version-min=">, Alias<mtvos_version_min_EQ>;
 def mtvos_simulator_version_min_EQ : Joined<["-"], "mtvos-simulator-version-min=">;
 def mappletvsimulator_version_min_EQ : Joined<["-"], "mappletvsimulator-version-min=">, Alias<mtvos_simulator_version_min_EQ>;
 def mwatchos_version_min_EQ : Joined<["-"], "mwatchos-version-min=">, Group<m_Group>;
 def mwatchos_simulator_version_min_EQ : Joined<["-"], "mwatchos-simulator-version-min=">;
 def mwatchsimulator_version_min_EQ : Joined<["-"], "mwatchsimulator-version-min=">, Alias<mwatchos_simulator_version_min_EQ>;
-def march_EQ : Joined<["-"], "march=">, Group<m_Group>, Flags<[CoreOption]>;
+def march_EQ : Joined<["-"], "march=">, Group<m_Group>, Flags<[CoreOption]>,
+  HelpText<"For a list of availible architectures for the target use '-mcpu=help'">;
 def masm_EQ : Joined<["-"], "masm=">, Group<m_Group>, Flags<[NoXarchOption]>;
 def inline_asm_EQ : Joined<["-"], "inline-asm=">, Group<m_Group>, Flags<[CC1Option]>,
   Values<"att,intel">,
   NormalizedValuesScope<"CodeGenOptions">, NormalizedValues<["IAD_ATT", "IAD_Intel"]>,
   MarshallingInfoEnum<CodeGenOpts<"InlineAsmDialect">, "IAD_ATT">;
 def mcmodel_EQ : Joined<["-"], "mcmodel=">, Group<m_Group>, Flags<[CC1Option]>,
   MarshallingInfoString<TargetOpts<"CodeModel">, [{"default"}]>;
 def mtls_size_EQ : Joined<["-"], "mtls-size=">, Group<m_Group>, Flags<[NoXarchOption, CC1Option]>,
   HelpText<"Specify bit size of immediate TLS offsets (AArch64 ELF only): "
            "12 (for 4KB) | 24 (for 16MB, default) | 32 (for 4GB) | 48 (for 256TB, needs -mcmodel=large)">,
   MarshallingInfoInt<CodeGenOpts<"TLSSize">>;
 def mimplicit_it_EQ : Joined<["-"], "mimplicit-it=">, Group<m_Group>;
 def mdefault_build_attributes : Joined<["-"], "mdefault-build-attributes">, Group<m_Group>;
 def mno_default_build_attributes : Joined<["-"], "mno-default-build-attributes">, Group<m_Group>;
 def mconstant_cfstrings : Flag<["-"], "mconstant-cfstrings">, Group<clang_ignored_m_Group>;
 def mconsole : Joined<["-"], "mconsole">, Group<m_Group>, Flags<[NoXarchOption]>;
 def mwindows : Joined<["-"], "mwindows">, Group<m_Group>, Flags<[NoXarchOption]>;
 def mdll : Joined<["-"], "mdll">, Group<m_Group>, Flags<[NoXarchOption]>;
 def municode : Joined<["-"], "municode">, Group<m_Group>, Flags<[NoXarchOption]>;
 def mthreads : Joined<["-"], "mthreads">, Group<m_Group>, Flags<[NoXarchOption]>;
 def mguard_EQ : Joined<["-"], "mguard=">, Group<m_Group>, Flags<[NoXarchOption]>,
   HelpText<"Enable or disable Control Flow Guard checks and guard tables emission">,
   Values<"none,cf,cf-nochecks">;
-def mcpu_EQ : Joined<["-"], "mcpu=">, Group<m_Group>;
+def mcpu_EQ : Joined<["-"], "mcpu=">, Group<m_Group>, 
+  HelpText<"For a list of availible CPUs for the target use '-mcpu=help'">;
 def mmcu_EQ : Joined<["-"], "mmcu=">, Group<m_Group>;
 def msim : Flag<["-"], "msim">, Group<m_Group>;
 def mdynamic_no_pic : Joined<["-"], "mdynamic-no-pic">, Group<m_Group>;
 def mfix_and_continue : Flag<["-"], "mfix-and-continue">, Group<clang_ignored_m_Group>;
 def mieee_fp : Flag<["-"], "mieee-fp">, Group<clang_ignored_m_Group>;
 def minline_all_stringops : Flag<["-"], "minline-all-stringops">, Group<clang_ignored_m_Group>;
 def mno_inline_all_stringops : Flag<["-"], "mno-inline-all-stringops">, Group<clang_ignored_m_Group>;
 def malign_double : Flag<["-"], "malign-double">, Group<m_Group>, Flags<[CC1Option]>,