Add support for all built-in text macros supported by ML64:
@Date, @Time, @FileName, @FileCur, and @CurSeg.
Reviewed By: thakis
Differential Revision: https://reviews.llvm.org/D104965
Also adds support for the ML.exe command-line flag /X, which ignores the INCLUDE environment variable.
This relands commit c43f413b01 using lit's cross-platform `env` support.
Differential Revision: https://reviews.llvm.org/D103989
Define -fatal-warnings to make warnings fatal, and accept /WX as an ML.EXE compatible alias for it.
Also make sure that if Warning() returns true, we always treat it as an error.
Reviewed By: thakis
Differential Revision: https://reviews.llvm.org/D92504