Use LLVM's new error handler API to report back end errors using Diagnostic.

For example,
--
ddunbar@giles:Frontend$ clang -c backend-errors.c
fatal error: error in backend: Global variable 'a' has an invalid section specifier
      'I AM, not, legal': mach-o section specifier uses an unknown section type.
--
compare to:
--
ddunbar@giles:Frontend$ gcc -c backend-errors.c
/var/folders/DQ/DQ8GT3++HESEzT1obWBynE+++TI/-Tmp-//cc45w2pq.s:2:Expected comma after segment-name
/var/folders/DQ/DQ8GT3++HESEzT1obWBynE+++TI/-Tmp-//cc45w2pq.s:2:Rest of line ignored. 1st junk character valued 77 (M).
--

Yay!

I am not tied to my wording choice, we could also go with "uncoverable error"
for the prefix, or just leave it off entirely.

llvm-svn: 78554

clang/include/clang/Basic/DiagnosticFrontendKinds.td

@@ -13,6 +13,7 @@ def err_fe_unknown_triple : Error<
   "unknown target triple '%0', please use -triple or -arch">;
 def err_fe_error_reading : Error<"error reading '%0'">;
 def err_fe_error_reading_stdin : Error<"error reading stdin">;
+def err_fe_error_backend : Error<"error in backend: %0">, DefaultFatal;
 
 def note_fixit_applied : Note<"FIX-IT applied suggested code changes">;
 def note_fixit_in_macro : Note<

clang/tools/clang-cc/clang-cc.cpp

@@ -58,6 +58,7 @@
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Config/config.h"
 #include "llvm/Support/CommandLine.h"
+#include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/ManagedStatic.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/PluginLoader.h"
@@ -2130,6 +2131,15 @@ static void ProcessInputFile(Preprocessor &PP, PreprocessorFactory &PPF,
 static llvm::cl::list<std::string>
 InputFilenames(llvm::cl::Positional, llvm::cl::desc("<input files>"));
 
+static void LLVMErrorHandler(void *UserData, const std::string &Message) {
+  Diagnostic &Diags = *static_cast<Diagnostic*>(UserData);
+
+  Diags.Report(FullSourceLoc(), diag::err_fe_error_backend) << Message;
+
+  // We cannot recover from llvm errors.
+  exit(1);
+}
+
 int main(int argc, char **argv) {
   llvm::sys::PrintStackTraceOnErrorSignal();
   llvm::PrettyStackTraceProgram X(argc, argv);
@@ -2208,6 +2218,11 @@ int main(int argc, char **argv) {
                             OptNoWarnings))
     return 1;
 
+  // Set an error handler, so that any LLVM backend diagnostics go through our
+  // error handler.
+  llvm::llvm_install_error_handler(LLVMErrorHandler,
+                                   static_cast<void*>(&Diags));
+
   // -I- is a deprecated GCC feature, scan for it and reject it.
   for (unsigned i = 0, e = I_dirs.size(); i != e; ++i) {
     if (I_dirs[i] == "-") {