MCContext::reportError: don't call report_fatal_error

Errors from MCAssembler, MCObjectStreamer and *ObjectWriter typically cause a crash:

```
% cat c.c
int bar;
extern int foo __attribute__((alias("bar")));
% clang -c -fcommon c.c
fatal error: error in backend: Common symbol 'bar' cannot be used in assignment expr
PLEASE submit a bug report to ...
Stack dump:
...
```

`LLVMTargetMachine::addPassesToEmitFile` constructs `MachineModuleInfoWrapperPass`
which creates a MCContext without SourceMgr. `MCContext::reportError` calls
`report_fatal_error` which gets captured by Clang `LLVMErrorHandler` and gets translated
to the output above.

Since `MCContext::reportError` errors indicate user errors, such a crashing style error
is inappropriate. So this patch changes `report_fatal_error` to `SourceMgr().PrintMessage`.
```
% clang -c -fcommon c.c
<unknown>:0: error: Common symbol 'bar' cannot be used in assignment expr
```

Ideally we should at least recover the original filename (the line information
is generally lost).  That requires general improvement to MC diagnostics,
because currently in many cases SMLoc information is lost.

1 files changed, 2 insertions, 2 deletions

diff --git a/llvm/lib/MC/MCContext.cpp b/llvm/lib/MC/MCContext.cpp
index fe84283..d054c93 100644
--- a/llvm/lib/MC/MCContext.cpp
+++ b/llvm/lib/MC/MCContext.cpp
@@ -825,13 +825,13 @@ void MCContext::reportError(SMLoc Loc, const Twine &Msg) {
 
   // If we have a source manager use it. Otherwise, try using the inline source
   // manager.
-  // If that fails, use the generic report_fatal_error().
+  // If that fails, construct a temporary SourceMgr.
   if (SrcMgr)
     SrcMgr->PrintMessage(Loc, SourceMgr::DK_Error, Msg);
   else if (InlineSrcMgr)
     InlineSrcMgr->PrintMessage(Loc, SourceMgr::DK_Error, Msg);
   else
-    report_fatal_error(Msg, false);
+    SourceMgr().PrintMessage(Loc, SourceMgr::DK_Error, Msg);
 }
 
 void MCContext::reportWarning(SMLoc Loc, const Twine &Msg) {