Pass EC by reference to MemoryBufferMMapFile to return error code.

Patch by Kim Grasman!

llvm-svn: 224159
diff --git a/llvm/lib/Support/MemoryBuffer.cpp b/llvm/lib/Support/MemoryBuffer.cpp
index 385ed6a..379db88 100644
--- a/llvm/lib/Support/MemoryBuffer.cpp
+++ b/llvm/lib/Support/MemoryBuffer.cpp
@@ -203,7 +203,7 @@
 
 public:
   MemoryBufferMMapFile(bool RequiresNullTerminator, int FD, uint64_t Len,
-                       uint64_t Offset, std::error_code EC)
+                       uint64_t Offset, std::error_code &EC)
       : MFR(FD, sys::fs::mapped_file_region::readonly,
             getLegalMapSize(Len, Offset), getLegalMapOffset(Offset), EC) {
     if (!EC) {