Exception handling docs: Describe landingpad clauses' meanings in more detail
The original text is very terse, so I've expanded on it.
Specifically, in the original text:
* "The selector value is a positive number if the exception matched a
type info" -- It wasn't clear that this meant "if the exception
matched a 'catch' clause".
* "If nothing is matched, the behavior of the program is
`undefined`_." -- It's actually implementation-defined in C++
rather than undefined, as the new text explains.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202209
91177308-0d34-0410-b5e6-
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