Added support for reading configuration files

Configuration file is read as a response file in which file names in
the nested constructs `@file` are resolved relative to the directory
where the including file resides. Lines in which the first non-whitespace
character is '#' are considered as comments and are skipped. Trailing
backslashes are used to concatenate lines in the same way as they
are used in shell scripts.

Differential Revision: https://reviews.llvm.org/D24926

llvm-svn: 321580