[asan] Optimize strchr for strict_string_checks=false

Summary:
strchr interceptor does not need to call strlen if strict_string_checks is not
enabled. Unnecessary strlen calls affect python parser performance.

Reviewers: eugenis, kcc

Subscribers: llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D32264

llvm-svn: 301027