[CMake][Compiler-rt] Compute `LLVM_MAIN_SRC_DIR` assuming the monorepo

layout.

When doing a standalone compiler-rt build we currently rely on
getting information from the `llvm-config` binary. Previously
we would rely on calling `llvm-config --src-root` to find the
LLVM sources. Unfortunately the returned path could easily be wrong
if the sources were built on another machine.

Now that compiler-rt is part of a monorepo we can easily fix this
problem by finding the LLVM source tree next to `compiler-rt` in
the monorepo. We do this regardless of whether or not the `llvm-config`
binary is available which moves us one step closer to not requiring
`llvm-config` to be available.

To try avoid anyone breaking anyone who relies on the current behavior,
if the path assuming the monorepo layout doesn't exist we invoke
`llvm-config --src-root` to get the path. A deprecation warning is
emitted if this path is taken because we should remove this path
in the future given that other runtimes already assume the monorepo
layout.

We also now emit a warning if `LLVM_MAIN_SRC_DIR` does not exist.
The intention is that this should be a hard error in future but
to avoid breaking existing users we'll keep this as a warning
for now.

rdar://76016632

Differential Revision: https://reviews.llvm.org/D99620