[libc++] Add additional benchmark functions to libcxx/benchmarks/string.bench

This change adds the following benchmarks:

- StringAssignStr
Assign a const basic::string& value

- StringAssignAsciiz
Assign a const char* asciiz value

StringAssignAsciizMix
Assign mixed long/short const char* asciiz values

- StringResizeDefaultInit
Resize default init benchmark

Patch by Martijn Vels (mvels@google.com)
Reviewed as D72343