visualstudio.py: Apply /utf-8 only on clang or VS2015+

In PR 10263, we didn't account for that we may have initialize the Visual
Studio-like compiler two times, once for a C compiler and once for the
C++ compiler, so we end up with Meson breaking on Visual Studio 2013
or earlier, such as when building GLib.

Fix this by setting up the always_args member of
the VisualStudioLikeCompiler instance during __init__() as needed, so that
we avoid falling into modifying shared objects.

mesonbuild/compilers/mixins/visualstudio.py

@@ -101,9 +101,7 @@ class VisualStudioLikeCompiler(Compiler, metaclass=abc.ABCMeta):
 
     # /showIncludes is needed for build dependency tracking in Ninja
     # See: https://ninja-build.org/manual.html#_deps
-    # Assume UTF-8 sources by default, but self.unix_args_to_native() removes it
-    # if `/source-charset` is set too.
-    always_args = ['/nologo', '/showIncludes', '/utf-8']
+    always_args = ['/nologo', '/showIncludes']  # type: T.List[str]
     warn_args = {
         '0': [],
         '1': ['/W2'],
@@ -117,6 +115,10 @@ class VisualStudioLikeCompiler(Compiler, metaclass=abc.ABCMeta):
         self.base_options = {mesonlib.OptionKey(o) for o in ['b_pch', 'b_ndebug', 'b_vscrt']} # FIXME add lto, pgo and the like
         self.target = target
         self.is_64 = ('x64' in target) or ('x86_64' in target)
+        # Assume UTF-8 sources by default on Visual Studio 2015 or later, or clang,
+        # but self.unix_args_to_native() removes it if `/source-charset` is set too.
+        if isinstance(self, ClangClCompiler) or mesonlib.version_compare(self.version, '>=19.00'):
+            self.always_args = self.always_args + ['/utf-8']
         # do some canonicalization of target machine
         if 'x86_64' in target:
             self.machine = 'x64'
@@ -130,10 +132,6 @@ class VisualStudioLikeCompiler(Compiler, metaclass=abc.ABCMeta):
             self.machine = target
         if mesonlib.version_compare(self.version, '>=19.28.29910'): # VS 16.9.0 includes cl 19.28.29910
             self.base_options.add(mesonlib.OptionKey('b_sanitize'))
-        # Exclude /utf-8 in self.always_args in VS2013 and earlier
-        if not isinstance(self, ClangClCompiler):
-            if mesonlib.version_compare(self.version, '<19.00'):
-                self.always_args.remove('/utf-8')
         assert self.linker is not None
         self.linker.machine = self.machine