[Modules] Soft reset diagnostics when clang module import failed
Swift compiler can try to import clang modules which might fail to load.
In this case, the clang diagnostic engine is reset to ignore the errors,
but the hard reset also clears out the states, including the diagnostics
options and current states. This will affect the future clang module
imports and diagnostics. Use a soft reset that only clears the errors
but not the other states.
rdar://139723218
(cherry picked from commit 04f09d6d1df6c4c0b70f6df4d50b895062857bdf)
