[preprocessor] When preprocessor option 'SingleFileParseMode' is enabled,...

[preprocessor] When preprocessor option 'SingleFileParseMode' is enabled, parse all directive blocks if the condition uses undefined macros

This is useful for being able to parse the preprocessor directive blocks even if the header, that defined the macro that is checked, hasn't been included.

Differential Revision: https://reviews.llvm.org/D34263

llvm-svn: 305797