Bug#1121657: dpkg-dev/dpkg-buildpackage doesn't check the Build-Profiles syntax

Package: src:dpkg
Version: 1.22.21
Severity: important
Tags: sid forky

As you asked to file a separate bug report:

dpkg-dev/dpkg-buildpackage doesn't check the Build-Profiles syntax and 
lets a package build fail.

llvm-toolchain-21 had used a syntax

Build-Profiles: <!pkg.llvm.noclang> + <!pkg.llvm.nolld>

which disabled the build of the binary packages having such a profile. 
dpkg-buildpackage doesn't complain about that syntax.  It should, and 
fail the build instead of just not building packages with such a profile.