Add -fsplit-lto-unit flag

The new LLVM update requires '-fsplit-lto-unit' to build ThinLTO. Add the flag unconditionally in test_compiler as a temporary workaround.

To be reverted after the cflag is properly set in soong.

Test: TreeHugger
Change-Id: I5b4dd4a7486c8219473d24a4b92447fb8994db75

test_compiler.py

diff --git a/test_compiler.py b/test_compiler.py
index 50ec8ce..947c7cc 100755
--- a/test_compiler.py
+++ b/test_compiler.py
@@ -35,6 +35,8 @@
 # before we fix those warnings.
 DISABLED_WARNINGS = [
     '-Wno-error=defaulted-function-deleted',
+    '-Wno-error=string-plus-int',
+    '-fsplit-lto-unit',
 ]