[XRay][compiler-rt][NFC] Add example always/never instrument files.

[dberris]

Summary

This change introduces two files that show exaples of the
always/never instrument files that can be provided to clang. We don't
add these as defaults yet in clang, which we can do later on (in a
separate change).

We also add a test that makes sure that these apply in the compiler-rt
project tests, and that changes in clang don't break the expectations in
compiler-rt.

[pelikan]

LGTM although I haven't build-tested it.

[pelikan]

Actually, maybe setting the threshold to 0 wouldn't be a bad idea.

lib/xray/xray_always_instrument.txt

+# List of function matchers common to C/C++ applications that make sense to
+# always instrument. You can use this as an argument to
+# -fxray-always-instrument=<path> along with your project-specific lists.
+
+# Always instrument the main function.
+fun:main

lib/xray/xray_never_instrument.txt

+# List of function matchers common to C/C++ applications that make sense to
+# never instrument. You can use this as an argument to
+# -fxray-never-instrument=<path> along with your project-specific lists.
+
+# Never instrument any function whose symbol starts with __xray.
+fun:__xray*

test/xray/TestCases/always-never-instrument.cc

+// Test that the always/never instrument lists apply.
+// RUN: echo "fun:main" > %tmp-always.txt
+// RUN: echo "fun:__xray*" > %tmp-never.txt
+// RUN: %clangxx_xray \
+// RUN:     -fxray-never-instrument=%tmp-never.txt \
+// RUN:     -fxray-always-instrument=%tmp-always.txt \
+// RUN:     %s -o %t
+// RUN: %llvm_xray extract -symbolize %t | \
+// RUN:    FileCheck %s --check-prefix NOINSTR
+// RUN: %llvm_xray extract -symbolize %t | \
+// RUN:    FileCheck %s --check-prefix ALWAYSINSTR
+
+// NOINSTR-NOT: {{.*__xray_NeverInstrumented.*}}
+int __xray_NeverInstrumented() {
+  return 0;
+}
+
+// ALWAYSINSTR: {{.*function-name:.*main.*}}
+int main(int argc, char *argv[]) {
+  return __xray_NeverInstrumented();
+}