[test] add test features static_stdlib

Add a test feature flag static_stdlib, which is enabled if the user
builds the static libraries.

We also add the substitution %target-static-stdlib-path which gives the
path of the static stdlib.

These features allow testing specifically for the static standard
library.

Author: drewcrawford

docs/Testing.rst

@@ -259,6 +259,10 @@ code for the target that is not the build machine:
 * ``%target-sdk-name``: only for Apple platforms: ``xcrun``-style SDK name
   (``macosx``, ``iphoneos``, ``iphonesimulator``).
 
+* ``%target-static-stdlib-path``: the path to the static standard library.
+
+  Add ``REQUIRES: static_stdlib`` to the test.
+
 Always use ``%target-*`` substitutions unless you have a good reason.  For
 example, an exception would be a test that checks how the compiler handles
 mixing module files for incompatible platforms (that test would need to compile

test/lit.cfg

@@ -871,3 +871,11 @@ config.substitutions.append(('%target-resilience-test', config.target_resilience
 
 config.substitutions.append(('%llvm-profdata', config.llvm_profdata))
 config.substitutions.append(('%llvm-cov', config.llvm_cov))
+
+# If static stdlib is present, enable static stdlib tests
+static_stdlib_path = os.path.join(os.path.join(config.swift_lib_dir,"swift_static"), config.target_sdk_name)
+libswiftCore_path = os.path.join(static_stdlib_path,"libswiftCore.a")
+if os.path.exists(libswiftCore_path):
+    config.available_features.add("static_stdlib")
+    config.substitutions.append(('%target-static-stdlib-path', static_stdlib_path))
+    lit_config.note('using static stdlib path: %s' % static_stdlib_path)