[clang][deps] Allow switching between lazily/eagerly loaded PCMs

This patch introduces new option `-eager-load-pcm` to `clang-scan-deps`, which controls whether the resulting command-lines will load PCM files eagerly (at the start of compilation) or lazily (when handling import directive). This patch also switches the default from eager to lazy.

To reduce the potential for churn in LIT tests in the future, this patch also removes redundant checks of command-line arguments and introduces new test `modules-dep-args.c` as a substitute.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D132066