rename the <predefines> buffer to <built-in> to solve PR3849.

Add a #include directive around the command line buffer so that
diagnostics generated from -include directives get diagnostics
like:

In file included from <built-in>:98:
In file included from <command line>:3:
./t.h:2:1: warning: type specifier missing, defaults to 'int'
b;
^

llvm-svn: 67396