It's pretty simple and works reliably as specified.
I can only infer that your lack of familiarity was what made it take so long.
Rebuilding GCC with specs does take forever, and building GCC is in general quite painful, but you could also use patchelf to modify the binary after the fact (which is what a lot of build systems do).
Clang cannot bootstrap in the same way GCC can; you need GCC (or another clang) to build it. You can obviously build it twice to have it be built by itself (bear in mind some of the clang components already do this, because they have to be built by clang).
In general though, a clang install will still depend on libstdc++, libgcc, GCC crtbegin.o and binutils (at least on Linux), which is typically why it will refer to a specific GCC install even after being built.
There are of course ways to use clang without any GCC runtime, but that's more involved and non-standard (unless you're on Mac).
And there is also the libc dependency (and all sysroot aspects in general) and while that is usually considered completely separate from GCC, the filesystem location and how it is found is often tied to how GCC is configured.
I can only infer that your lack of familiarity was what made it take so long.
Rebuilding GCC with specs does take forever, and building GCC is in general quite painful, but you could also use patchelf to modify the binary after the fact (which is what a lot of build systems do).