[Tooling] FixedCompilationDatabase should be able to strip positional

arguments when `-fsyntax-only` is used

Previously, Clang failed to create a fixed compilation database when the
compilation arguments use -fsyntax-only instead of -c. This commit fixes the
issue by forcing Clang to look at the compilation job when stripping the
positional arguments.

Differential Revision: https://reviews.llvm.org/D34687

llvm-svn: 306659