MSVC: Disable buggy conformance check

/Zc:lambda seems buggy. Although in my experiments it works well
for 99% Qt repos, it seems some tests will trigger the bug and it
also blocks some new commits. So disable it for now, it's not stable
enough.

Now that this check is disabled, the workaround for tst_qstringapisymmetry
is also not needed anymore, so remove the workaround as well.

Partially reverts commit 8cb832090a

Change-Id: Icf0ecbbaa6262522470e5f5dea05705985ab18f1
Reviewed-by: Jörg Bornemann <joerg.bornemann@qt.io>

cmake/QtInternalTargets.cmake

@@ -248,7 +248,7 @@ if (MSVC)
     if (MSVC_VERSION GREATER_EQUAL 1919 AND NOT CLANG) # MSVC 2019
         target_compile_options(PlatformCommonInternal INTERFACE
             -Zc:externConstexpr
-            -Zc:lambda
+            #-Zc:lambda # Buggy. TODO: Enable again when stable enough.
             -Zc:preprocessor
         )
     endif()

tests/auto/corelib/text/qstringapisymmetry/tst_qstringapisymmetry.cpp

@@ -2626,7 +2626,7 @@ void tst_QStringApiSymmetry::toNumberWithBases_data()
     QTest::addColumn<qint64>("result");
     QTest::addColumn<bool>("ok");
 
-    static constexpr struct {
+    constexpr struct {
         const char prefix[3];
         int base;
     } bases[] = {