[libcxx] Add code coverage configuration to CMake and LIT.

Summary:
This patch adds configuration to CMake and LIT for running the libc++ test-suite to generate code coverage.

To use code coverage use following instructions.

* Find the clang resource dir using `$CXX -print-search-dirs`. Let <library-dir> be the first library search directory.
* `cmake <regular-options> -DLIBCXX_GENERATE_COVERAGE=ON -DLIBCXX_COVERAGE_LIBRARY=<library-dir>/lib/<platform>/libclang_rt.profile.a <source>`
* `make cxx`
* `make check-libcxx`
* `make generate-libcxx-coverage`


The reason I want this patch upstreamed is so I can setup a bot that generates code coverage and posts in online for every revision. 



Reviewers: mclow.lists, jroelofs, danalbert

Reviewed By: danalbert

Differential Revision: http://reviews.llvm.org/D8716

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@233669 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/test/config.py

@@ -98,6 +98,7 @@ class Configuration(object):
         self.configure_debug_mode()
         self.configure_warnings()
         self.configure_sanitizer()
+        self.configure_coverage()
         self.configure_substitutions()
         self.configure_features()
 
@@ -594,6 +595,12 @@ class Configuration(object):
                 self.lit_config.fatal('unsupported value for '
                                       'use_sanitizer: {0}'.format(san))
 
+    def configure_coverage(self):
+        self.generate_coverage = self.get_lit_bool('generate_coverage', False)
+        if self.generate_coverage:
+            self.cxx.flags += ['-g', '--coverage']
+            self.cxx.compile_flags += ['-O0']
+
     def configure_substitutions(self):
         sub = self.config.substitutions
         # Configure compiler substitions