[XRay][compiler-rt+docs] Introduce __xray_log_init_mode(...).
[XRay][compiler-rt+docs] Introduce __xray_log_init_mode(...).

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

The change Deprecates a number of functions and types in
`include/xray/xray_log_interface.h` to recommend using string-based
configuration of XRay through the __xray_log_init_mode(...) function. In
particular, this deprecates the following:

- `__xray_set_log_impl(...)` -- users should instead use the
`__xray_log_register_mode(...)` and `__xray_log_select_mode(...)` APIs.

- `__xray_log_init(...)` -- users should instead use the
`__xray_log_init_mode(...)` function, which also requires using the
`__xray_log_register_mode(...)` and `__xray_log_select_mode(...)`
functionality.

- `__xray::FDRLoggingOptions` -- in following patches, we'll be
migrating the FDR logging implementations (and tests) to use the
string-based configuration. In later stages we'll remove the
`__xray::FDRLoggingOptions` type, and ask users to migrate to using the
string-based configuration mechanism instead.

- `__xray::BasicLoggingOptions` -- same as `__xray::FDRLoggingOptions`,
we'll be removing this type later and instead rely exclusively on the
string-based configuration API.

We also update the documentation to reflect the new advice and remove
some of the deprecated notes.

Reviewers: eizan, kpw, echristo, pelikan

Reviewed By: kpw

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D46173

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@331503 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm