Turn on -utf-8 compiler option for msvc2015 update 2 and up

Building qt with msvc would fail with the -developer-build configure
option turned on under simplified chinese locale.
This is because msvc will emit warnings for source files with utf-8
characters which are not representable in CP936, and -developer-build
implies treating warnings as errors.
This patch turn on -utf-8 compiler option for msvc2015 update 2
and up only for building qt.

Task-number: QTBUG-58161
Change-Id: If38ea11eb1f39f8e08efa1cccb92e0eea50daf92
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

mkspecs/common/msvc-version.conf

@@ -66,6 +66,12 @@ greaterThan(QMAKE_MSC_VER, 1899) {
     QMAKE_CFLAGS_AVX2   = -arch:AVX2
     QMAKE_CXXFLAGS     += -Zc:strictStrings -Zc:throwingNew
     QMAKE_CXXFLAGS_WARN_ON += -w44456 -w44457 -w44458 -wd4577 -wd4467
+
+    greaterThan(QMAKE_MSC_FULL_VER, 190023918):!intel_icl {
+        isEmpty(QT_CLANG_MAJOR_VERSION)|!lessThan(QT_CLANG_MAJOR_VERSION, 4) {
+            QMAKE_CFLAGS_UTF8_SOURCE = -utf-8
+        }
+    }
 }
 
 greaterThan(QMAKE_MSC_VER, 1909) {

mkspecs/features/default_post.prf

@@ -116,6 +116,11 @@ c++11|c++14|c++1z {
     unset(cxxstd)
 }
 
+utf8_source {
+    QMAKE_CFLAGS += $$QMAKE_CFLAGS_UTF8_SOURCE
+    QMAKE_CXXFLAGS += $$QMAKE_CFLAGS_UTF8_SOURCE
+}
+
 !precompile_header: SOURCES += $$NO_PCH_SOURCES
 
 QMAKE_INCDIR += $$QMAKE_INCDIR_POST

mkspecs/features/qt_build_config.prf

@@ -84,6 +84,7 @@ android|uikit|winrt: \
     CONFIG += builtin_testdata
 
 CONFIG += \
+    utf8_source \
     create_prl link_prl \
     prepare_docs qt_docs_targets \
     no_private_qt_headers_warning QTDIR_build \