Add support for variant_prepend properties in bp2build for cc library

Some properties in soong are marked "variant_prepend", which means
in bp2build output, variant properties(select ...) should come before
general properties.
Prepend property is added in StringListAttribute
stuct so that bp2build can handlle variant_prepend properly, if
Prepend is properly set in different module logics.
This change also coordinates cc library with Prepend.
Coordination with other module types will be in separate CLs.

Bug: 261644490
Test: TestCcLibraryHeadersSimple,
TestCcLibraryHeadersArchAndTargetExportSystemIncludes and TH

Change-Id: I3b61c491c54bbe91ba9aa2af9b4c84193c91ae7f
diff --git a/bp2build/cc_library_headers_conversion_test.go b/bp2build/cc_library_headers_conversion_test.go
index 7d9db6f..686c9d5 100644
--- a/bp2build/cc_library_headers_conversion_test.go
+++ b/bp2build/cc_library_headers_conversion_test.go
@@ -107,15 +107,15 @@
 }`,
 		ExpectedBazelTargets: []string{
 			MakeBazelTarget("cc_library_headers", "foo_headers", AttrNameToString{
-				"export_includes": `[
-        "dir-1",
-        "dir-2",
-    ] + select({
+				"export_includes": `select({
         "//build/bazel/platforms/arch:arm64": ["arch_arm64_exported_include_dir"],
         "//build/bazel/platforms/arch:x86": ["arch_x86_exported_include_dir"],
         "//build/bazel/platforms/arch:x86_64": ["arch_x86_64_exported_include_dir"],
         "//conditions:default": [],
-    })`,
+    }) + [
+        "dir-1",
+        "dir-2",
+    ]`,
 				"sdk_version":     `"current"`,
 				"min_sdk_version": `"29"`,
 			}),
@@ -340,16 +340,16 @@
 }`,
 		ExpectedBazelTargets: []string{
 			MakeBazelTarget("cc_library_headers", "foo_headers", AttrNameToString{
-				"export_system_includes": `["shared_include_dir"] + select({
-        "//build/bazel/platforms/arch:arm": ["arm_include_dir"],
-        "//build/bazel/platforms/arch:x86_64": ["x86_64_include_dir"],
-        "//conditions:default": [],
-    }) + select({
+				"export_system_includes": `select({
         "//build/bazel/platforms/os:android": ["android_include_dir"],
         "//build/bazel/platforms/os:darwin": ["darwin_include_dir"],
         "//build/bazel/platforms/os:linux": ["linux_include_dir"],
         "//conditions:default": [],
-    })`,
+    }) + select({
+        "//build/bazel/platforms/arch:arm": ["arm_include_dir"],
+        "//build/bazel/platforms/arch:x86_64": ["x86_64_include_dir"],
+        "//conditions:default": [],
+    }) + ["shared_include_dir"]`,
 			}),
 		},
 	})