[obj2yaml] - Stop triggering UB when dumping corrupted strings.

We have a following code to find quote type:

if (isspace(S.front()) || isspace(S.back()))
...

Problem is that:

"int isspace( int ch ): The behavior is undefined if the value of
ch is not representable as unsigned char and is not equal to EOF."
(https://en.cppreference.com/w/cpp/string/byte/isspace)

This patch shows how this UB can be triggered and fixes an issue.

Differential revision: https://reviews.llvm.org/D69160

llvm-svn: 375404