Add support for passing -main-file-name all the way through to
the assembler.

Part of PR14624

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@170390 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/MC/MCContext.cpp b/lib/MC/MCContext.cpp
index 29727ac..d206dd9 100644
--- a/lib/MC/MCContext.cpp
+++ b/lib/MC/MCContext.cpp
@@ -21,6 +21,7 @@
 #include "llvm/MC/MCSymbol.h"
 #include "llvm/Support/ELF.h"
 #include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/SourceMgr.h"
 using namespace llvm;
@@ -48,6 +49,11 @@
   SecureLogFile = getenv("AS_SECURE_LOG_FILE");
   SecureLog = 0;
   SecureLogUsed = false;
+
+  if (SrcMgr && SrcMgr->getNumBuffers() > 0)
+    MainFileName = SrcMgr->getMemoryBuffer(0)->getBufferIdentifier();
+  else
+    MainFileName = "";
 }
 
 MCContext::~MCContext() {