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

Summary:
In this chage we add support for the string-based configuration
mechanism for configuring FDR mode.

We deprecate most of the `xray_fdr_log_*` flags that are set with the
`XRAY_OPTIONS` environment variable. Instead we make the FDR
implementation take defaults from the `XRAY_FDR_OPTIONS` environment
variable, and use the flags defined in `xray_fdr_flags.{h,cc,inc}` for
the options we support.

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

Depends on D46173.

Reviewers: eizan, pelikan, kpw, echristo

Subscribers: llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D46174

llvm-svn: 331506