VS: Fix VS 2015 .vcxproj debug setting for older toolsets (#15986)

Since commit v3.4.2~2^2 (VS: Fix VS 2015 .vcxproj file value for
GenerateDebugInformation, 2016-01-08) we generate invalid project
files for the v110 and v120 toolsets.  VS complains:

    C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\V120\Microsoft.CppCommon.targets(639,9):
    error MSB4030: "Debug" is an invalid value for the "GenerateDebugInformation" parameter of
    the "Link" task. The "GenerateDebugInformation" parameter is of type "System.Boolean".

This reveals that our VS flag map selection should be based on the
toolset instead of the version of VS.  However, that will be a
non-trivial change so for now fix this particular use case by
hard-coding a correction to the flag map.

Reported-by: Gregor Jasny <gjasny@googlemail.com>