COMP: Suppress MSVC C4750 dashboard warning from vendored Eigen3

Tridiagonalization.h emits C4750 ('function with _alloca() inlined into
a loop') under MSVC whenever an ITK consumer instantiates the symmetric
eigen-decomposition path.  The warning is benign (Eigen's intentional
small-allocation alloca path) but cannot be addressed inside ITK and
fails CDash because the dashboard treats any warning as fatal.

The previous attempt to set CMAKE_CXX_FLAGS in the vendored Eigen
CMakeLists was a no-op (Eigen is header-only, so no .cxx file picks up
the flag) and the alternative -- attaching /wd4750 to the eigen_internal
INTERFACE library -- would propagate to ITKCommon (which DEPENDS on
ITKEigen3) and from there to most of the toolkit, suppressing C4750
for non-Eigen ITK code too.  Both rejected per @blowekamp.

Instead, extend the existing 'Modules/ThirdParty/Eigen3/.*warning:.*'
CTestCustom regex with a sibling that matches MSVC's
'warning C####:' form.  Compiler still emits the warning so reviewers
see it in build logs, but the dashboard ignores it for warning-count
purposes -- exactly the same scope the GCC/Clang side has had since
this exception was added.

Author: hjmjohnson

CMake/CTestCustom.cmake.in

@@ -132,6 +132,10 @@ set(CTEST_CUSTOM_WARNING_EXCEPTION
   # ignore some third party warnings
   ".*Modules/ThirdParty/.*warning:.*Wzero-as-null-pointer-constant"
   ".*Modules/ThirdParty/Eigen3/.*warning:.*"
+  # MSVC emits "warning C####:" rather than "warning:"; cover that form too
+  # so the Tridiagonalization.h C4750 ('_alloca() inlined into a loop') and
+  # similar Eigen-internal MSVC warnings do not fail the dashboard.
+  ".*Modules.ThirdParty.Eigen3.*warning C[0-9]+:.*"
 
   # CircleCI distcc warnings
   ".*WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED.*"