Enhancement to test for -ast-print

Present tests for the functionality provided by command lime option
`-ast-print` check only absence of crash. This change tries to make
testing better, - the output produced by the compiler is compiled again
with option `-print-ast` and both outputs are compared. Such test at
least checks that the output is valid code. This change fixes only the
test for pure C.

llvm-svn: 285882