These generate a warning when included, then include the old header.
Note for GCC, #warning is used; this requires -std=gnu++14 instead of
-std=c++14 (otherwise the warning is treated as an error because #warning is
a GNU extension). On MSVC, #pragma message is used, which is a bit
unsatisfactory as the message doesn't say where it was included from.
The llvm shim headers also include a llvm namespace shim.
During shared library loading, a different libLLVM can be pulled in, causing
llvm symbols from dependent libraries to resolve to that library instead of
this one. This has been seen in the wild with the Mesa OpenGL implementation
in JavaFX applications (see wpilibsuite/shuffleboard#361).
This is clearly a very breaking change. For some level of backwards
compatibility, a namespace alias from llvm to wpi is performed in the "llvm"
headers. Unfortunately, forward declarations of llvm classes will still break,
but compilers seem to generate clear error messages in those cases
("namespace alias 'llvm' not allowed here, assuming 'wpi'").
This change also moves all the wpiutil headers to a single "wpi" subdirectory
from the previously split "llvm", "support", "tcpsockets", and "udpsockets".
Shim headers will be added for backwards compatibility in a later commit.
