[llvm] [unittest] Allow getting a C string from the TempDir helper class

The TempDir.path() member function returns a StringRef. We've been
calling the data() method on that StringRef, which does not guarantee
to return a null-terminated string (required by chdir and other POSIX
functions).

Introduce the c_str() method in the TempDir class, which returns the
proper string without the need to create a copy of the path at use site.

llvm/include/llvm/Testing/Support/SupportHelpers.h

@@ -152,6 +152,9 @@ public:
   /// The path to the temporary directory.
   StringRef path() const { return Path; }
 
+  /// The null-terminated C string pointing to the path.
+  const char *c_str() { return Path.c_str(); }
+
   /// Creates a new path by appending the argument to the path of the managed
   /// directory using the native path separator.
   SmallString<128> path(StringRef component) const {

llvm/unittests/Support/LockFileManagerTest.cpp

@@ -81,7 +81,7 @@ TEST(LockFileManagerTest, RelativePath) {
 
   char PathBuf[1024];
   const char *OrigPath = getcwd(PathBuf, 1024);
-  ASSERT_FALSE(chdir(LockFileManagerTestDir.path().data()));
+  ASSERT_FALSE(chdir(LockFileManagerTestDir.c_str()));
 
   TempDir inner("inner");
   SmallString<64> LockedFile(inner.path());