Give the extra compiler a unique name for the vcproj generation

The first word of the .name variable of each extra compiler is
used as a key in a container that keeps track of them. See also:
qmake/generators/win32/msvc_objectmodel.cpp: VCProjectSingleConfig::filterForExtraCompiler
qmake/generators/win32/msvc_vcproj.cpp: VcprojGenerators::initExtraCompilerOutputs

Task-number: QTBUG-32912
Change-Id: I7ea5c58884db559621f50740075b7f2e4e3ef7f8
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>