Remove variables exported by Soong

LLVM_RELEASE_VERSION is unused from envsetup.sh and during product
config loading.  LLVM_PREBUILTS_VERSION is still used in envsetup.sh, so
needs to stay.

Delay reading core/clang/config.mk until after Soong's variables are
read so that we don't need a copy of LLVM_PREBUILTS_BASE.

Test: Compare build-aosp_flounder.ninja
Change-Id: I85b5836a2230838c8f3766fb19c6108fc5c9ca50
diff --git a/core/main.mk b/core/main.mk
index 5a3042a..77b213c 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -50,6 +50,8 @@
 
 include $(SOONG_MAKEVARS_MK)
 
+include $(BUILD_SYSTEM)/clang/config.mk
+
 # Write the build number to a file so it can be read back in
 # without changing the command line every time.  Avoids rebuilds
 # when using ninja.