[flang] More Fortran runtime support for CHARACTER operations

Summary:
- Remove C++ library dependence from lock.h
- Implement LEN_TRIM, REPEAT, ADJUSTL, ADJUSTR, MAX/MIN
  intrinsic functions for CHARACTER

Reviewers: tskeith, PeteSteinfeld, sscalpone, schweitz, DavidTruby

Reviewed By: PeteSteinfeld

Subscribers: llvm-commits, flang-commits

Tags: #flang, #llvm

Differential Revision: https://reviews.llvm.org/D82054