[libc] Clean up required LIBC_INLINE uses in src/string

This was generated using clang-tidy and clang-apply-replacements,
on src/string/*.cpp for just the llvmlibc-inline-function-decl
check, after applying https://reviews.llvm.org/D157164, and then
some manual fixup.

Reviewed By: abrachet

Differential Revision: https://reviews.llvm.org/D157169