[lldb] Preserve original symbol of Mangled function names (llvm#152201)

Fixes a bug that surfaces in frame recognizers.

Details about the bug:

A new frame recognizer is configured to match a specific symbol
(`swift_willThrow`). This is an `extern "C"` symbol defined in a C++
source file. When Swift is built with debug info, the function
`ParseFunctionFromDWARF` will use the debug info to construct a function
name that looks like a C++ declaration (`::swift_willThrow(void *,
SwiftError**)`). The `Mangled` instance will have this string as its
`m_demangled` field, and have _no_ string for its `m_mangled` field.

The result is the frame recognizer would not match the symbol to the
name (`swift_willThrow` != `::swift_willThrow(void *, SwiftError**)`.

By changing `ParseFunctionFromDWARF` to assign both a demangled name and
a mangled, frame recognizers can successfully match symbols in this
configuration.

(cherry picked from commit 2959051)

Author: kastiglione

lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp

@@ -2543,7 +2543,9 @@ DWARFASTParserClang::ParseFunctionFromDWARF(CompileUnit &comp_unit,
       // If the mangled name is not present in the DWARF, generate the
       // demangled name using the decl context. We skip if the function is
       // "main" as its name is never mangled.
-      func_name.SetValue(ConstructDemangledNameFromDWARF(die));
+      func_name.SetDemangledName(ConstructDemangledNameFromDWARF(die));
+      // Ensure symbol is preserved (as the mangled name).
+      func_name.SetMangledName(ConstString(name));
     } else
       func_name.SetValue(ConstString(name));
 

lldb/test/API/lang/cpp/extern_c/main.cpp

@@ -8,8 +8,10 @@ extern "C"
 
 int foo()
 {
-    puts("foo");
-    return 2;
+  puts("foo"); //% self.expect("image lookup -va $pc",
+  //%                          substrs=[' name = "::foo()"',
+  //%                                   ' mangled = "foo"'])
+  return 2;
 }
 
 int main (int argc, char const *argv[], char const *envp[])

lldb/test/Shell/SymbolFile/DWARF/x86/debug-types-address-ranges.s

@@ -11,7 +11,7 @@
 # RUN: %lldb %t -o "image lookup -a 0x48000 -v" -o exit | FileCheck %s
 
 # CHECK:   CompileUnit: id = {0x00000001}, file = "/tmp/a.cc", language = "c++"
-# CHECK:      Function: id = {0x0000006a}, name = "::_start({{.*}})", range = [0x0000000000048000-0x000000000004800c)
+# CHECK:      Function: id = {0x0000006a}, name = "::_start({{.*}})", mangled = "_start", range = [0x0000000000048000-0x000000000004800c)
 # CHECK:     LineEntry: [0x0000000000048000-0x000000000004800a): /tmp/a.cc:4
 # CHECK:        Symbol: id = {0x00000002}, range = [0x0000000000048000-0x000000000004800c), name="_start"
 # CHECK:      Variable: id = {0x00000075}, name = "v1", {{.*}} decl = a.cc:4