[libc++][NFC] Mark values in gdb pretty print comparison functions as…

… live to prevent values being optimized out.

It appears when testing LLVM 13 on Power, we run into failures with the
`libcxx/test/libcxx/gdb/gdb_pretty_printer_test.sh.cpp` test case optimizing
values out.

Despite some the functions in the test already being marked with optnone,
adding the `MarkAsLive()` calls inside of the pretty printer comparison functions
resolves the issues of the values being optimized out.

This patch aims to address https://llvm.org/PR51675.

Differential Revision: https://reviews.llvm.org/D109204

(cherry picked from commit 217c6d6)

libcxx/test/libcxx/gdb/gdb_pretty_printer_test.sh.cpp

@@ -92,24 +92,28 @@ void MarkAsLive(Type &&) {}
 template <typename TypeToPrint> void ComparePrettyPrintToChars(
     TypeToPrint value,
     const char *expectation) {
+  MarkAsLive(value);
   StopForDebugger(&value, &expectation);
 }
 
 template <typename TypeToPrint> void ComparePrettyPrintToRegex(
     TypeToPrint value,
     const char *expectation) {
+  MarkAsLive(value);
   StopForDebugger(&value, &expectation);
 }
 
 void CompareExpressionPrettyPrintToChars(
     std::string value,
     const char *expectation) {
+  MarkAsLive(value);
   StopForDebugger(&value, &expectation);
 }
 
 void CompareExpressionPrettyPrintToRegex(
     std::string value,
     const char *expectation) {
+  MarkAsLive(value);
   StopForDebugger(&value, &expectation);
 }