msvc: handle filename extensions for incdetect based on the compiler language

It is possible, albeit possibly inadvisable, for the exact combination
of MSVC and "$CXX has C++ specific flags in it" to occur. When this
happens, and cl.exe is given a filename ending in .c, it complains that
you cannot compile a .c file with that option.

Instead, pick the first filename matching that language and use that as
the temporary filename. This more or less matches what we do in
compiler-time checks. And it's the proper thing to do, rather than
assume that cl.exe, when detected as the current C++ compiler, can
*also* compile C because it's *also* a C compiler.

Fixes #11257

mesonbuild/backend/ninjabackend.py

@@ -523,8 +523,9 @@ class NinjaBackend(backends.Backend):
         else:
             # None of our compilers are MSVC, we're done.
             return open(tempfilename, 'a', encoding='utf-8')
+        filebase = 'incdetect.' + compilers.lang_suffixes[compiler.language][0]
         filename = os.path.join(self.environment.get_scratch_dir(),
-                                'incdetect.c')
+                                filebase)
         with open(filename, 'w', encoding='utf-8') as f:
             f.write(dedent('''\
                 #include<stdio.h>
@@ -536,7 +537,7 @@ class NinjaBackend(backends.Backend):
         # Python strings leads to failure. We _must_ do this detection
         # in raw byte mode and write the result in raw bytes.
         pc = subprocess.Popen(compiler.get_exelist() +
-                              ['/showIncludes', '/c', 'incdetect.c'],
+                              ['/showIncludes', '/c', filebase],
                               cwd=self.environment.get_scratch_dir(),
                               stdout=subprocess.PIPE, stderr=subprocess.PIPE)
         (stdout, stderr) = pc.communicate()