More aggressively deduplicate global warnings based on contents. (#112801)

I've been getting complaints from users being spammed by -gmodules
missing file warnings going out of control because each object file
depends on an entire DAG of PCM files that usually are all missing at
once. To reduce this problem, this patch does two things:

1. Module now maintains a DenseMap<hash, once> that is used to display
each warning only once, based on its actual text.

2. The PCM warning itself is reworded to include less details, such as
the DIE offset, which is only useful to LLDB developers, who can get
this from the dwarf log if they need it. Because the detail is omitted
the hashing from (1) deduplicates the warnings.

rdar://138144624

0 files changed, 0 insertions, 0 deletions