Enable incremental parsing by the Preprocessor,
where more code can be provided after an EOF.
It mainly prevents the tearing down of the topmost lexer.
To be used like this:
PP.enableIncrementalProcessing();
while (getMoreSource()) {
  while (Parser.ParseTopLevelDecl(ADecl)) {...}
}
PP.enableIncrementalProcessing(false);


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@152914 91177308-0d34-0410-b5e6-96231b3b80d8