Configure build mode for LIT tests. (#190)

This change allows us to query the build mode used to build the compiler
in LIT tests. This will enable the REQUIRES constraint to disable tests
when the compiler is not built with asserts or in debug mode. See
https://llvm.org/docs/TestingGuide.html\#constraining-test-execution for
more details on constraining test execution.

Author: kitbarton

test/lit.cfg.py

@@ -55,3 +55,7 @@
 # Add substitutions for all files in QSS compiler tools directory.
 tools = os.listdir(config.qss_compiler_tools_dir)
 llvm_config.add_tool_substitutions(tools, [config.qss_compiler_tools_dir])
+
+llvm_config.feature_config(
+    [('--assertion-mode', {'ON': 'asserts'}),
+     ('--build-mode', {'[Dd][Ee][Bb][Uu][Gg]': 'debug'})])