Restrict the set of plugins used for ProcessMinidump

1. The dynamic loaders should not be needed for loading minidumps
and they may create problems (ex. the macOS loader resets the list of
loaded sections, which for minidumps are already set up during minidump loading)

2. In general, the extra plugins can do extraneous work which hurts performance
(ex. trying to set up implicit symbolic breakpoints, which in turn will trigger
extra debug information loading)

Differential Revision: https://reviews.llvm.org/D51176

llvm-svn: 340578