build fail -- clang-11 not found

Hi everyone, I recently forked and cloned upstream llvm-project/ (IIRC
the tests were passing on upstream at the time I cloned). I used the
following cmake command for my debug build.

    -DCMAKE_C_COMPILER=/usr/bin/clang \
    -DCMAKE_CXX_COMPILER=/usr/bin/clang++ \

Followed by `ninja -j8`, it threw me errors saying "clang-11 not found",
and "use -v for invocations" as suggestion; despite clang-11 being
present in my system and PATH.

One more thing, it threw errors while building libLTO.

Can someone help me out? I have never encountered this.

Thank you

PS: Sorry I dont have the exact error.