Reland: Optimize Google Test process startup

Google Test performs hidden test registration during process
startup. For test binaries that contain a large number of tests, this
registration can be costly. In this CL, we reduce the overhead of
registration via several tactics:

- Treat CodeLocation and FilePath as value types, using std::move to
  pass them around.
- Reduce string copies in various places by either passing std::string
  values via std::move, or passing const-refs to std::string instances.
- Use std::to_string to stringify an int in DefaultParamName rather than
  a std::stringstream.
- Pull some std::string instances out of nested loops in
  ParameterizedTestSuiteInfo::RegisterTests so as to reuse some
  allocations, and replace stringstream with ordinary string appends.
- Use std::unordered_map in UnitTestImpl::GetTestSuite and
  ParameterizedTestSuiteRegistry::GetTestSuitePatternHolder to spend a
  little memory to turn O(N) lookups into constant time lookpus.
- Use range-based for loops in a few places.
- Use emplace-ish methods to add to containers where appropriate.

All together, these changes reduce the overall runtime of a series of 50
death tests in a single Chromium test executable by ~38% due to the
fact that the registration costs are paid in every death test's child
process.

PiperOrigin-RevId: 613833210
Change-Id: I51a262a770edff98ffa1e3b60c4d78a8308f9a9f

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