backends: Only add pch args that are appropriate for the compiler

Currently we try both C and C++ when determining which PCH files to
include. The problem with this approach is that if there are no C or C++
files (only headers) and the target has both C and C++ sources then the
PCHs will be passed to the wrong compiler.

The solution is less code, we already have the compiler, the compiler
knows what language it is, so we don't need to walk both C and C++.

Fixes #3068

mesonbuild/backend/backends.py

@@ -407,16 +407,10 @@ class Backend:
         args = []
         pchpath = self.get_target_private_dir(target)
         includeargs = compiler.get_include_args(pchpath, False)
-        for lang in ['c', 'cpp']:
-            p = target.get_pch(lang)
-            if not p:
-                continue
-            if compiler.can_compile(p[-1]):
-                header = p[0]
-                args += compiler.get_pch_use_args(pchpath, header)
-        if len(args) > 0:
-            args = includeargs + args
-        return args
+        p = target.get_pch(compiler.get_language())
+        if p:
+            args += compiler.get_pch_use_args(pchpath, p[0])
+        return includeargs + args
 
     @staticmethod
     def escape_extra_args(compiler, args):