Export the ASTBuilder class from the AST module.

llvm-svn: 39095
diff --git a/clang/Lex/MacroExpander.cpp b/clang/Lex/MacroExpander.cpp
index a7a645d..f51c308f 100644
--- a/clang/Lex/MacroExpander.cpp
+++ b/clang/Lex/MacroExpander.cpp
@@ -523,6 +523,7 @@
 
     // Allocate space for the result token.  This is guaranteed to be enough for
     // the two tokens and a null terminator.
+    // FIXME: Use a smallvector here.
     char *Buffer = (char*)alloca(Tok.getLength() + RHS.getLength() + 1);
     
     // Get the spelling of the LHS token in Buffer.