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
diff --git a/cc/config/global.go b/cc/config/global.go
index 359ea8c..989c7ee 100644
--- a/cc/config/global.go
+++ b/cc/config/global.go
@@ -116,8 +116,8 @@
 
 	// prebuilts/clang default settings.
 	ClangDefaultBase         = "prebuilts/clang/host"
-	ClangDefaultVersion      = "clang-4639204"
-	ClangDefaultShortVersion = "6.0.1"
+	ClangDefaultVersion      = "clang-4679922"
+	ClangDefaultShortVersion = "7.0.1"
 
 	// Directories with warnings from Android.bp files.
 	WarningAllowedProjects = []string{