Use filename in linemarker when compiling preprocessed source

Summary:
Clang appears to always use name as specified on the command
line, whereas gcc uses the name as specified in the linemarker at the
first line when compiling a preprocessed source. This results mismatch
between two compilers in FILE symbol table entry. This patch makes clang
to resemble gcc's behavior in finding the original source file name and
use it as an input file name.

Even with this patch, values of FILE symbol table entry may still be
different because clang uses dirname+basename for the entry whlie gcc
uses basename only. I'll write a patch for that once this patch is
committed.

Reviewers: dblaikie, inglorion

Reviewed By: inglorion

Subscribers: inglorion, aprantl, bruno

Differential Revision: https://reviews.llvm.org/D28796

llvm-svn: 293004