[clang-offload-bundler] Add option -allow-missing-bundles

There are out-of-tree tools using clang-offload-bundler to extract
bundles from bundled files. When a bundle is not in the bundled
file, clang-offload-bundler is expected to emit an error message
and return non-zero value. However currently clang-offload-bundler
silently generates empty file for the missing bundles.

Since OpenMP/HIP toolchains expect the current behavior, an option
-allow-missing-bundles is added to let clang-offload-bundler
create empty file when a bundle is missing when unbundling.
The unbundling job action is updated to use this option by
default.

clang-offload-bundler itself will emit error when a bundle
is missing when unbundling by default.

Changes are also made to check duplicate targets in -targets
option and emit error.

Differential Revision: https://reviews.llvm.org/D93068