xcode: Set bridging header for mixed swift targets

This must be explicitly set in Xcode or it will not be able to compile
mixed targets successfully. This is not needed for pure Swift targets.

Set a new variable "is_swift" so finding the bridging header does not
take O(n^2) time.

Fixes passing swift test case 5 (mixed) with Xcode backend.

mesonbuild/backend/xcodebackend.py

@@ -1467,6 +1467,8 @@ class XCodeBackend(backends.Backend):
             dep_libs = []
             links_dylib = False
             headerdirs = []
+            bridging_header = ""
+            is_swift = self.is_swift_target(target)
             for d in target.include_dirs:
                 for sd in d.incdirs:
                     cd = os.path.join(d.curdir, sd)
@@ -1474,6 +1476,13 @@ class XCodeBackend(backends.Backend):
                     headerdirs.append(os.path.join(self.environment.get_build_dir(), cd))
                 for extra in d.extra_build_dirs:
                     headerdirs.append(os.path.join(self.environment.get_build_dir(), extra))
+            # Swift can import declarations from C-based code using bridging headers.
+            # There can only be one header, and it must be included as a source file.
+            for i in target.get_sources():
+                if self.environment.is_header(i) and is_swift:
+                    relh = i.rel_to_builddir(self.build_to_src)
+                    bridging_header = os.path.normpath(os.path.join(self.environment.get_build_dir(), relh))
+                    break
             (dep_libs, links_dylib) = self.determine_internal_dep_link_args(target, buildtype)
             if links_dylib:
                 dep_libs = ['-Wl,-search_paths_first', '-Wl,-headerpad_max_install_names'] + dep_libs
@@ -1496,7 +1505,7 @@ class XCodeBackend(backends.Backend):
             ldargs += target.link_args
             # Swift is special. Again. You can't mix Swift with other languages
             # in the same target. Thus for Swift we only use
-            if self.is_swift_target(target):
+            if is_swift:
                 linker, stdlib_args = target.compilers['swift'], []
             else:
                 linker, stdlib_args = self.determine_linker_and_stdlib_args(target)
@@ -1651,6 +1660,8 @@ class XCodeBackend(backends.Backend):
             else:
                 settings_dict.add_item('PRODUCT_NAME', product_name)
             settings_dict.add_item('SECTORDER_FLAGS', '""')
+            if is_swift and bridging_header:
+                settings_dict.add_item('SWIFT_OBJC_BRIDGING_HEADER', f'"{bridging_header}"')
             settings_dict.add_item('SYMROOT', f'"{symroot}"')
             sysheader_arr = PbxArray()
             # XCode will change every -I flag that points inside these directories