Allow breaking between a type and name in variable declarations.

This fixes llvm.org/PR14967 and is generall necessary to avoid
situations where the column limit is exceeded. The challenge is
restricting such lines splits, otherwise clang-format suddenly starts
breaking at bad places.

Before:
ReallyLongReturnType<TemplateParam1, TemplateParam2>
ReallyReallyLongFunctionName(
    const std::string &SomeParameter,
    const SomeType<string,
                   SomeOtherTemplateParameter> &ReallyReallyLongParameterName,
    const SomeType<string,
                   SomeOtherTemplateParameter> &AnotherLongParameterName) {}

After:
ReallyLongReturnType<TemplateParam1, TemplateParam2>
ReallyReallyLongFunctionName(
    const std::string &SomeParameter,
    const SomeType<string, SomeOtherTemplateParameter> &
        ReallyReallyLongParameterName,
    const SomeType<string, SomeOtherTemplateParameter> &
        AnotherLongParameterName) {}