[Support/GlobPattern] - Do not crash when pattern has characters with int value < 0.

Found it during work on LLD, it would crash on following 
linker script:

SECTIONS { .foo : { *("*®") } }
That happens because ® has int value -82. And chars are used as
array index in code, and are signed by default.

Differential revision: https://reviews.llvm.org/D35891

llvm-svn: 309549