Switch to clang 7.0

* Suppress more noisy new warnings at global level.
* Add -no-pie to partial link .o files, with -r.
* Revert workaround of b/72706604, no need of
  -Wl,-plugin-opt,-emulated-tls
* Filter out clang 7.0 unknown flag "-Wno-extended-offsetof"

Bug: 72706604
Bug: 72412006
Test: make checkbuild
Change-Id: I7ff45465c4bd771991f42b40f68dc35586045656

cc/builder.go

@@ -67,7 +67,9 @@ var (
 
 	partialLd = pctx.AndroidStaticRule("partialLd",
 		blueprint.RuleParams{
-			Command:     "$ldCmd -nostdlib -Wl,-r ${in} -o ${out} ${ldFlags}",
+			// Without -no-pie, clang 7.0 adds -pie to link Android files,
+			// but -r and -pie cannot be used together.
+			Command:     "$ldCmd -nostdlib -no-pie -Wl,-r ${in} -o ${out} ${ldFlags}",
 			CommandDeps: []string{"$ldCmd"},
 		},
 		"ldCmd", "ldFlags")