llvm-mc: Fix llvm-mc -as-lex.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@108644 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/tools/llvm-mc/llvm-mc.cpp b/tools/llvm-mc/llvm-mc.cpp
index fc8a1c5..34cf8ce 100644
--- a/tools/llvm-mc/llvm-mc.cpp
+++ b/tools/llvm-mc/llvm-mc.cpp
@@ -165,9 +165,9 @@
   assert(MAI && "Unable to create target asm info!");
 
   AsmLexer Lexer(*MAI);
-  
+  Lexer.setBuffer(SrcMgr.getMemoryBuffer(0));
+
   bool Error = false;
-  
   while (Lexer.Lex().isNot(AsmToken::Eof)) {
     switch (Lexer.getKind()) {
     default: