[Clang][Driver] Change missing multilib error to warning

[michaelplatings]

Summary

The error could be awkward to work around when experimenting with flags
that didn't have a matching multilib. It also broke many tests when
multilib.yaml was present in the build directory.

[simon_tatham]

clang/include/clang/Basic/DiagnosticDriverKinds.td

740–742

Tiny clarity nit: any chance of renaming this identifier to warn_drv_missing_multilib, while we're renaming it anyway? Then the command-line option to control the warning matches the identifier inside the code, and makes it easier to guess the spelling of either one if you had the other one already in mind.

[michaelplatings]

Rename variable

clang/include/clang/Basic/DiagnosticDriverKinds.td

def warn_drv_loongarch_conflicting_implied_val : Warning<
   "ignoring '%0' as it conflicts with that implied by '%1' (%2)">,
   InGroup<OptionIgnored>;
 def err_drv_loongarch_invalid_mfpu_EQ : Error<
   "invalid argument '%0' to -mfpu=; must be one of: 64, 32, none, 0 (alias for none)">;
 
 def err_drv_expand_response_file : Error<
   "failed to expand response file: %0">;
 
-def err_drv_no_matching_multilib : Error<
-  "no multilib found matching flags: %0">;
+def warn_drv_missing_multilib : Warning<
+  "no multilib found matching flags: %0">,
+  InGroup<DiagGroup<"missing-multilib">>;
 def note_drv_available_multilibs : Note<
   "available multilibs are:%0">;
 }

clang/lib/Driver/ToolChains/BareMetal.cpp

static void findMultilibsFromYAML(const ToolChain &TC, const Driver &D,
   Multilib::flags_list Flags = TC.getMultilibFlags(Args);
   llvm::ErrorOr<MultilibSet> ErrorOrMultilibSet =
       MultilibSet::parseYaml(*MB.get());
   if (ErrorOrMultilibSet.getError())
     return;
   Result.Multilibs = ErrorOrMultilibSet.get();
   if (Result.Multilibs.select(Flags, Result.SelectedMultilibs))
     return;
-  D.Diag(clang::diag::err_drv_no_matching_multilib) << llvm::join(Flags, " ");
+  D.Diag(clang::diag::warn_drv_missing_multilib) << llvm::join(Flags, " ");
   std::stringstream ss;
   for (const Multilib &Multilib : Result.Multilibs)
     ss << "\n" << llvm::join(Multilib.flags(), " ");
   D.Diag(clang::diag::note_drv_available_multilibs) << ss.str();
 }
 
 static constexpr llvm::StringLiteral MultilibFilename = "multilib.yaml";
 

clang/test/Driver/baremetal-multilib.yaml

 # CHECK-NEXT: ld{{(.exe)?}}" "{{.*}}.o" "-Bstatic"
 # CHECK-SAME: "-L[[SYSROOT]]/bin/../lib/clang-runtimes/arm-none-eabi/thumb/v8-m.main/fp/lib"
 # CHECK-SAME: "-lc" "-lm" "-lclang_rt.builtins"
 # CHECK-SAME: "-o" "{{.*}}.tmp.out"
 
 # RUN: %T/baremetal_multilib/bin/clang -no-canonical-prefixes -x c++ %s -### -o %t.out 2>&1 \
 # RUN:     --target=thumbv7em-none-eabi -mfpu=fpv4-sp-d16 --sysroot= \
 # RUN:   | FileCheck --check-prefix=CHECK-NO-MATCH %s
-# CHECK-NO-MATCH: error: no multilib found matching flags:
+# CHECK-NO-MATCH: warning: no multilib found matching flags:
 # CHECK-NO-MATCH-SAME: --target=thumbv7em-none-unknown-eabi
 # CHECK-NO-MATCH: note: available multilibs are:
 # CHECK-NO-MATCH: --target=thumbv6m-none-unknown-eabi -mfpu=none
 # CHECK-NO-MATCH: --target=thumbv7m-none-unknown-eabi -mfpu=none
 # CHECK-NO-MATCH: --target=thumbv7em-none-unknown-eabi -mfpu=none
 
 # RUN: %T/baremetal_multilib/bin/clang -no-canonical-prefixes -print-multi-directory 2>&1 \
 # RUN:     --target=thumbv8m.main-none-eabihf --sysroot= \