[clang-scan-deps] Allow continuation line backslashes followed by whitespace

in the dependency source minimizer

Clang allows continuations that have whitespace between the backslash and the newline.
This patch ensures that the dependency source minimizer can handle the whitespace between
the backslash and the newline when looking for a line continuation.

Differential Revision: https://reviews.llvm.org/D68052

llvm-svn: 373007