Build: Move TF source file inclusion from build system to source files

Build: Move TF source file inclusion from build system to source files

Outside of compiler-rt (where it's arguably an anti-pattern too),
LLVM tries to keep its build files as simple as possible. See e.g.
llvm/docs/SupportLibrary.rst, "Code Organization".

Differential Revision: https://reviews.llvm.org/D84243