[DependencyScanning] Fix warnings
This patch fixes:
clang/lib/Tooling/DependencyScanning/DependencyScannerImpl.cpp:733:10:
error: unused variable 'ArgSize' [-Werror,-Wunused-variable]
clang/lib/Tooling/DependencyScanning/DependencyScannerImpl.cpp:735:15:
error: unused variable 'FirstArg' [-Werror,-Wunused-variable]
