Remove leaking UnknownPragmaHandlers right after we are done with them.

The UnknownPragmaHandlers added by DoPrintPreprocessedInput conflict with the
real PragmaHandlers from clang::Parser because they try to handle the same
#pragma directives. This makes it impossible to use a Preprocessor (that was
previously passed to DoPrintPreprocessedInput), as an Preprocessor for a
clang::Parser instance which is what we currently do in cling.

This patch removes the added UnknownPragmaHandler to avoid conflicts these
conflicts and leave the PragmaHandlers of the Preprocessors in a the same state
as before calling DoPrintPreprocessedInput.

Patch by Raphael Isemann (D32486)!

llvm-svn: 301563