[XRay][clang+compiler-rt] Support build-time mode selection

Summary:
This patch implements the `-fxray-modes=` flag which allows users
building with XRay instrumentation to decide which modes to pre-package
into the binary being linked. The default is the status quo, which will
link all the available modes.

For this to work we're also breaking apart the mode implementations
(xray-fdr and xray-basic) from the main xray runtime. This gives more
granular control of which modes are pre-packaged, and picked from
clang's invocation.

This fixes llvm.org/PR37066.

Note that in the future, we may change the default for clang to only
contain the profiling implementation under development in D44620, when
that implementation is ready.

Reviewers: echristo, eizan, chandlerc

Reviewed By: echristo

Subscribers: mgorny, mgrang, cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D45474

llvm-svn: 329772