[Driver][BareMetal] Error if no matching multilib

Previously if no matching multilib was found then the user would
typically see an error like "fatal error: 'stdio.h' file not found"
which gives no indication as to the underlying problem.
With this change the user will instead see an error like
  clang: error: no multilib found matching flags: --target=thumbv7em-none-unknown-eabi -march=...
  clang: note: available multilibs are:
  --target=armv4t-none-unknown-eabi
  --target=thumbv6m-none-unknown-eabi -mfpu=none
  ...

Differential Revision: https://reviews.llvm.org/D153292

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -733,4 +733,9 @@ def err_drv_loongarch_invalid_mfpu_EQ : Error<
 
 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 note_drv_available_multilibs : Note<
+  "available multilibs are:%0">;
 }

clang/lib/Driver/ToolChains/BareMetal.cpp

@@ -23,6 +23,8 @@
 #include "llvm/Support/VirtualFileSystem.h"
 #include "llvm/Support/raw_ostream.h"
 
+#include <sstream>
+
 using namespace llvm::opt;
 using namespace clang;
 using namespace clang::driver;
@@ -158,20 +160,26 @@ static bool isRISCVBareMetal(const llvm::Triple &Triple) {
   return Triple.getEnvironmentName() == "elf";
 }
 
-static bool findMultilibsFromYAML(const ToolChain &TC, const Driver &D,
+static void findMultilibsFromYAML(const ToolChain &TC, const Driver &D,
                                   StringRef MultilibPath, const ArgList &Args,
                                   DetectedMultilibs &Result) {
   llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> MB =
       D.getVFS().getBufferForFile(MultilibPath);
   if (!MB)
-    return false;
+    return;
   Multilib::flags_list Flags = TC.getMultilibFlags(Args);
   llvm::ErrorOr<MultilibSet> ErrorOrMultilibSet =
       MultilibSet::parseYaml(*MB.get());
   if (ErrorOrMultilibSet.getError())
-    return false;
+    return;
   Result.Multilibs = ErrorOrMultilibSet.get();
-  return Result.Multilibs.select(Flags, Result.SelectedMultilibs);
+  if (Result.Multilibs.select(Flags, Result.SelectedMultilibs))
+    return;
+  D.Diag(clang::diag::err_drv_no_matching_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

@@ -20,6 +20,16 @@
 # 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-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= \
 # RUN:   | FileCheck --check-prefix=CHECK-PRINT-MULTI-DIRECTORY %s
@@ -75,6 +85,9 @@ MultilibVersion: 1.0
 # multilib, layered on top of each other.
 
 Variants:
+- Dir: arm-none-eabi/arm/v4t
+  Flags: [--target=armv4t-none-unknown-eabi]
+
 - Dir: arm-none-eabi/thumb/v6-m/nofp
   Flags: [--target=thumbv6m-none-unknown-eabi, -mfpu=none]