clang-format: fix fallback style set to "none" not always formatting

This fixes clang-format not formatting if fallback-style is explicitly set to
"none", and either a config file is found or YAML is passed in without a
"BasedOnStyle". With this change, passing "none" in these cases will have no
affect, and LLVM style will be used as the base style.

Differential Revision: https://reviews.llvm.org/D28844

llvm-svn: 292562