[lldb][swig] Support SBFileSpec::GetPath (llvm#162964)

# Summary
`SBFileSpec::GetPath(char *dst_path, size_t dst_len)` contains `char*`
type argument. Need to handle this for python. Fortunately there're
already similar definitions we can reuse.

# Test Plan
Start the freshly built lldb and run the following code
```
$ lldb
(lldb) script
Python Interactive Interpreter. To exit, type 'quit()', 'exit()' or Ctrl-D.
>>> debugger = lldb.SBDebugger.Create()
>>> debugger.SetAsync (False)
>>> target = debugger.CreateTarget("~/tmp/hello")
>>> target.IsValid()
True
>>> breakpoint = target.BreakpointCreateByName('main', 'hello')
>>> breakpoint.GetNumLocations()
1
>>> process = target.LaunchSimple (None, None, os.getcwd())
>>> process.IsValid()
True
>>> thread = process.GetThreadAtIndex(0)
>>> frame = thread.GetFrameAtIndex(0)
>>> line = frame.GetLineEntry()

# Important line below
>>> file = line.GetFileSpec().GetPath(1024)
# Important line above

>>> print(file)
/home/wanyi/tmp/main.cpp
```

## Before this change
```
$ lldb
(lldb) script
Python Interactive Interpreter. To exit, type 'quit()', 'exit()' or Ctrl-D.
>>> debugger = lldb.SBDebugger.Create()
>>> debugger.SetAsync (False)
>>> target = debugger.CreateTarget("~/tmp/hello")
>>> target.IsValid()
True
>>> breakpoint = target.BreakpointCreateByName('main', 'hello')
>>> breakpoint.GetNumLocations()
1
>>> process = target.LaunchSimple (None, None, os.getcwd())
>>> process.IsValid()
True
>>> thread = process.GetThreadAtIndex(0)
>>> frame = thread.GetFrameAtIndex(0)
>>> line = frame.GetLineEntry()
>>> file = line.GetFileSpec().GetPath(1024)
Traceback (most recent call last):
  File "<console>", line 1, in <module>
TypeError: SBFileSpec.GetPath() missing 1 required positional argument: 'dst_len'
>>> print(file)
Traceback (most recent call last):
  File "<console>", line 1, in <module>
NameError: name 'file' is not defined
```

Author: kusmour

lldb/bindings/python/python-typemaps.swig

@@ -233,6 +233,11 @@ AND call SWIG_fail at the same time, because it will result in a double free.
 }
 
 
+// For lldb::SBFileSpec::GetPath
+%typemap(in) (char *dst_path, size_t dst_len) = (char *dst_or_null, size_t dst_len);
+%typemap(argout) (char *dst_path, size_t dst_len) = (char *dst_or_null, size_t dst_len);
+
+
 // typemap for an outgoing buffer
 // See also SBEvent::SBEvent(uint32_t event, const char *cstr, uint32_t cstr_len).
 // Ditto for SBProcess::PutSTDIN(const char *src, size_t src_len).

lldb/test/API/python_api/default-constructor/sb_filespec.py

@@ -10,5 +10,5 @@ def fuzz_obj(obj):
     obj.ResolveExecutableLocation()
     obj.GetFilename()
     obj.GetDirectory()
-    obj.GetPath(None, 0)
+    obj.GetPath(1)
     obj.GetDescription(lldb.SBStream())