Use the file name from linemarker for debug info if an input is preprocessed source.

Summary:
Even in the case of the input file is a preprocessed source, clang uses the file name of the preprocesses source for debug info (DW_AT_name attribute for DW_TAG_compile_unit). However, gcc uses the file name specified in the first linemarker instead. This makes more sense because the one specified in the linemarker represents the "actual" source file name.

Clang already uses the file name specified in the first linemarker for Module name (https://github.com/llvm-mirror/clang/blob/master/lib/Frontend/FrontendAction.cpp#L779) if the input is preprocessed. This patch makes clang to use the same value for debug info as well.

Reviewers: compnerd, rnk, dblaikie, rsmith

Reviewed By: rnk

Subscribers: aprantl, cfe-commits

Differential Revision: https://reviews.llvm.org/D36474

llvm-svn: 311037