[CodeGen] prevent abnormal on invalid attributes

Currently, when an invalid attribute is encountered on processing a .s file,
clang will abort due to llvm_unreachable.  Invalid user input should not cause
an abnormal termination of the compiler.  Change the interface to return a
boolean to indicate the failure as a first step towards improving hanlding of
malformed user input to clang.

Signed-off-by: Saleem Abdulrasool <compnerd@compnerd.org>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188047 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/MC/WinCOFFStreamer.cpp b/lib/MC/WinCOFFStreamer.cpp
index 04bfeb4..3f0449d 100644
--- a/lib/MC/WinCOFFStreamer.cpp
+++ b/lib/MC/WinCOFFStreamer.cpp
@@ -55,7 +55,7 @@
   virtual void EmitDebugLabel(MCSymbol *Symbol);
   virtual void EmitAssemblerFlag(MCAssemblerFlag Flag);
   virtual void EmitThumbFunc(MCSymbol *Func);
-  virtual void EmitSymbolAttribute(MCSymbol *Symbol, MCSymbolAttr Attribute);
+  virtual bool EmitSymbolAttribute(MCSymbol *Symbol, MCSymbolAttr Attribute);
   virtual void EmitSymbolDesc(MCSymbol *Symbol, unsigned DescValue);
   virtual void BeginCOFFSymbolDef(MCSymbol const *Symbol);
   virtual void EmitCOFFSymbolStorageClass(int StorageClass);
@@ -201,7 +201,7 @@
   llvm_unreachable("not implemented");
 }
 
-void WinCOFFStreamer::EmitSymbolAttribute(MCSymbol *Symbol,
+bool WinCOFFStreamer::EmitSymbolAttribute(MCSymbol *Symbol,
                                           MCSymbolAttr Attribute) {
   assert(Symbol && "Symbol must be non-null!");
   assert((Symbol->isInSection()
@@ -221,8 +221,10 @@
     break;
 
   default:
-    llvm_unreachable("unsupported attribute");
+    return false;
   }
+
+  return true;
 }
 
 void WinCOFFStreamer::EmitSymbolDesc(MCSymbol *Symbol, unsigned DescValue) {