[dfsan] Fix a bug in strcasecmp() and strncasecmp(): Compare the lowercase...

[dfsan] Fix a bug in strcasecmp() and strncasecmp(): Compare the lowercase versions of the characters when choosing a return value.

Summary:
Resolves this bug:

  https://bugs.llvm.org/show_bug.cgi?id=38369

Reviewers: morehouse, pcc

Reviewed By: morehouse

Subscribers: #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D78490