Minor fix to connect-url to support unix-connect sockets on localhost (#142875)

**Summary:**

when the unix-socket connections on localhost are used to for platform
connect i.e.
`platform connect unix-connect:///path/to/socket.sock`
then `PlatformRemoteGDBServer.m_platform_hostname` is empty.

Based on the current logic, for the process attach, when the connection
param returned by platform server as qLaunchGDBServer is this
`socket_name:/path/to/processgdbserver.sock`
then the subsequent connect url for the process url looks like this
`unix-connect://[]/path/to/processgdbserver.sock` and the connection
fail.

This change is only adding the braces when the hostname is not empty.

**Test Plan:**

Added unittest and existing tests pass.
```
satyajanga@devvm21837:toolchain $ LLDB_COMMAND_TRACE=YES ./bin/llvm-lit --verbose  ~/llvm-sand/external/llvm-project/lldb/test/API/commands/platform
-- Testing: 9 tests, 9 workers --
UNSUPPORTED: lldb-api :: commands/platform/sdk/TestPlatformSDK.py (1 of 9)
PASS: lldb-api :: commands/platform/file/read/TestPlatformFileRead.py (2 of 9)
PASS: lldb-api :: commands/platform/file/close/TestPlatformFileClose.py (3 of 9)
PASS: lldb-api :: commands/platform/basic/TestPlatformPython.py (4 of 9)
PASS: lldb-api :: commands/platform/basic/TestPlatformCommand.py (5 of 9)
PASS: lldb-api :: commands/platform/process/launch/TestPlatformProcessLaunch.py (6 of 9)
PASS: lldb-api :: commands/platform/connect/TestPlatformConnect.py (7 of 9)
PASS: lldb-api :: commands/platform/launchgdbserver/TestPlatformLaunchGDBServer.py (8 of 9)
PASS: lldb-api :: commands/platform/process/list/TestProcessList.py (9 of 9)

Testing Time: 13.24s

Total Discovered Tests: 9
  Unsupported: 1 (11.11%)
  Passed     : 8 (88.89%)
satyajanga@devvm21837:toolchain $ 

```


Reviewers:

@clayborg 
@Jlalond 

Subscribers:

Tasks:

Tags:

0 files changed, 0 insertions, 0 deletions