Add some support for pretty printing Twines containing std::string in gdb

This isn't perfect, since it doesn't use lazy_string - so if the
std::string does contain unprintable characters it might fail, but seems
better than nothing & LLVM doesn't generally store binary data in
std::strings.

cross-project-tests/debuginfo-tests/llvm-prettyprinters/gdb/llvm-support.cpp

@@ -27,8 +27,8 @@ llvm::StringRef StringRef = "bar";
 // Should test std::string in Twine too, but it's currently broken because I
 // don't know how to add 'str' and 'gdb.LazyString' (can't figure out any way to
 // string-ify LazyString).
-//std::string String = "foo";
-llvm::Twine TempTwine = llvm::Twine(3) + StringRef;
+std::string String = "foo";
+llvm::Twine TempTwine = llvm::Twine(String) + StringRef;
 llvm::Twine Twine = TempTwine + "baz";
 llvm::PointerIntPair<int *, 1> PointerIntPair(IntPtr, 1);
 

cross-project-tests/debuginfo-tests/llvm-prettyprinters/gdb/llvm-support.gdb

@@ -37,7 +37,7 @@ p SmallString
 # CHECK: "bar"
 p StringRef
 
-# CHECK: "3barbaz"
+# CHECK: "foobarbaz"
 p Twine
 
 # CHECK: llvm::StringMap with 2 elements = {["foo"] = 123, ["bar"] = 456}

llvm/utils/gdb-scripts/prettyprinters.py

@@ -271,7 +271,7 @@ class TwinePrinter:
       # register the LazyString type, so we can't check
       # "type(s) == gdb.LazyString".
       if 'LazyString' in type(s).__name__:
-        s = s.value().address.string()
+        s = s.value().string()
 
     else:
       print(('No pretty printer for {} found. The resulting Twine ' +