[Support] Promote cl::StringSaver to a separate utility

This class is generally useful.

In breaking it out, the primary change is that it has been made
non-virtual. It seems like being abstract led to there being 3 different
(2 in llvm + 1 in clang) concrete implementations which disagreed about
the ownership of the saved strings (see the manual call to free() in the
unittest StrDupSaver; yes this is different from the CommandLine.cpp
StrDupSaver which owns the stored strings; which is different from
Clang's StringSetSaver which just holds a reference to a
std::set<std::string> which owns the strings).

I've identified 2 other places in the
codebase that are open-coding this pattern:

  memcpy(Alloc.Allocate<char>(strlen(S)+1), S, strlen(S)+1)

I'll be switching them over. They are
* llvm::sys::Process::GetArgumentVector
* The StringAllocator member of YAMLIO's Input class
This also will allow simplifying Clang's driver.cpp quite a bit.

Let me know if there are any other places that could benefit from
StringSaver. I'm also thinking of adding a saveStringRef member for
getting a stable StringRef.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215784 91177308-0d34-0410-b5e6-96231b3b80d8

unittests/Support/CommandLineTest.cpp

@@ -146,26 +146,19 @@ TEST(CommandLineTest, UseOptionCategory) {
                                                   "Category.";
 }
 
-class StrDupSaver : public cl::StringSaver {
-  const char *SaveString(const char *Str) override {
-    return strdup(Str);
-  }
-};
-
-typedef void ParserFunction(StringRef Source, llvm::cl::StringSaver &Saver,
+typedef void ParserFunction(StringRef Source, StringSaver &Saver,
                             SmallVectorImpl<const char *> &NewArgv);
 
 
 void testCommandLineTokenizer(ParserFunction *parse, const char *Input,
                               const char *const Output[], size_t OutputSize) {
   SmallVector<const char *, 0> Actual;
-  StrDupSaver Saver;
+  StringSaver Saver;
   parse(Input, Saver, Actual);
   EXPECT_EQ(OutputSize, Actual.size());
   for (unsigned I = 0, E = Actual.size(); I != E; ++I) {
     if (I < OutputSize)
       EXPECT_STREQ(Output[I], Actual[I]);
-    free(const_cast<char *>(Actual[I]));
   }
 }