[XRay][compiler-rt] Support string-based config for Basic mode.

Summary:
This addresses http://llvm.org/PR36790.

This change allows the XRay Basic Mode implementation to use the
string-based initialization routine provided through
`__xray_log_init_mode(...)`. In the process, we've also deprecated some
flags defined for the `XRAY_OPTIONS` environment variable.

We then introduce another environment variable that can control the XRay
Basic Mode implementation through `XRAY_BASIC_OPTIONS`.

We also rename files from `xray_inmemory_log` to `xray_basic_logging` to
be more in line with the mode implementation.

Depends on D46174.

Reviewers: echristo, kpw, pelikan, eizan

Reviewed By: kpw

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D46246

llvm-svn: 331507