This has been an unnecessary code path for a long time now and should be removed particularly because it triggers wasteful `stat` calls.
rdar://51523161
Instead of appending a character for each substitution, we now prefix the substitution with the repeat count, e.g.
AbbbbB -> A5B
The same is done for known-type substitutions, e.g.
SiSiSi -> S3i
This significantly shrinks mangled names which contain large lists of the same type, like
func foo(_ x: (Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int, Int))
rdar://problem/30707433
After 7400d484 we tried to walk into enum elements, but forgot to check
for missing types (which caused an assertion in getType) or element
decls (which caused an assertion or crash inside passReference).
rdar://problem/24634223
