Upgrade to clang-r547379

Stop defining _LIBCPP_HAS_MUSL_LIBC (to 1) because it is now defined
(to blank) the musl __config_site headers.

Bug: http://b/379133546
Test: presubmit
Change-Id: I7f8fe4cb0e3b69d90b23fc3e63b7046e3607e092

diff --git a/cc/config/global.go b/cc/config/global.go
index a309dc0..19ed8e7 100644
--- a/cc/config/global.go
+++ b/cc/config/global.go
@@ -383,8 +383,8 @@
 
 	// prebuilts/clang default settings.
 	ClangDefaultBase         = "prebuilts/clang/host"
-	ClangDefaultVersion      = "clang-r536225"
-	ClangDefaultShortVersion = "19"
+	ClangDefaultVersion      = "clang-r547379"
+	ClangDefaultShortVersion = "20"
 
 	// Directories with warnings from Android.bp files.
 	WarningAllowedProjects = []string{
diff --git a/cc/config/x86_linux_host.go b/cc/config/x86_linux_host.go
index 287967c..c070050 100644
--- a/cc/config/x86_linux_host.go
+++ b/cc/config/x86_linux_host.go
@@ -41,7 +41,6 @@
 	}
 
 	linuxMuslCflags = []string{
-		"-D_LIBCPP_HAS_MUSL_LIBC",
 		"-DANDROID_HOST_MUSL",
 		"-nostdlibinc",
 		"--sysroot /dev/null",