When skipping code at the start of a file during PCH use, Preprocessor::Lex

is not used since it consumes all preprocessor directives until it returns
a real token. Using the specific Lexer (i.e. CurLexer->Lex) makes it
possible to stop skipping after an #include or #pragma hdrstop. Previously
the skipping code was only handling CurLexer, now all will be handled
correctly.

Fixes: llvm.org/PR41585

Differential Revision: https://reviews.llvm.org/D61217

llvm-svn: 359506