[LLDB] Don't print register fields when asked for a specific format

Previously if a register had fields we would always print them after the
value if the register was asked for by name.
```
(lldb) register read MDCR_EL3
MDCR_EL3 = 0x00000000
         = {
             ETBAD = 0
<...>
             RLTE = 0
           }
```
This can be quite annoying if there are a whole lot of fields but you
want to see the register in a specific format.
```
(lldb) register read MDCR_EL3 -f i
MDCR_EL3 = 0x00000000   unknown     udf    #0x0
         = {
             ETBAD = 0
<...lots of fields...>
```
Since it pushes the interesting bit far up the terminal. To solve this,
don't print fields if the user passes --format. If they're doing that
then I think it's reasonable to assume they know what they want and only
want to see that output.

This also gives users a way to silence fields, but not change the format.
By doing `register read foo -f x`. In case they are not useful or perhaps
they are trying to work around a crash.

I have customised the help text for --format for register read to explain this:
```
-f <format> ( --format <format> )
     Specify a format to be used for display. If this is set, register fields will not be dispayed.
```

Reviewed By: jasonmolenda

Differential Revision: https://reviews.llvm.org/D148790

lldb/source/Commands/CommandObjectRegister.cpp

@@ -44,7 +44,10 @@ public:
             nullptr,
             eCommandRequiresFrame | eCommandRequiresRegContext |
                 eCommandProcessMustBeLaunched | eCommandProcessMustBePaused),
-        m_format_options(eFormatDefault) {
+        m_format_options(eFormatDefault, UINT64_MAX, UINT64_MAX,
+                         {{CommandArgumentType::eArgTypeFormat,
+                           "Specify a format to be used for display. If this "
+                           "is set, register fields will not be displayed."}}) {
     CommandArgumentEntry arg;
     CommandArgumentData register_arg;
 
@@ -215,8 +218,12 @@ protected:
 
           if (const RegisterInfo *reg_info =
                   reg_ctx->GetRegisterInfoByName(arg_str)) {
+            // If they have asked for a specific format don't obscure that by
+            // printing flags afterwards.
+            bool print_flags =
+                !m_format_options.GetFormatValue().OptionWasSet();
             if (!DumpRegister(m_exe_ctx, strm, *reg_ctx, *reg_info,
-                              /*print_flags=*/true))
+                              print_flags))
               strm.Printf("%-12s = error: unavailable\n", reg_info->name);
           } else {
             result.AppendErrorWithFormat("Invalid register name '%s'.\n",

lldb/test/API/functionalities/gdb_remote_client/TestXMLRegisterFlags.py

@@ -410,6 +410,15 @@ class TestXMLRegisterFlags(GDBRemoteTestBase):
 
         self.expect("register read cpsr", substrs=["= (field_2 = 1, field_1 = 1, ...)"])
 
+    @skipIfXmlSupportMissing
+    @skipIfRemote
+    def test_format_disables_flags(self):
+        # If asked for a specific format, don't print flags after it.
+        self.setup_flags_test('<field name="field_0" start="0" end="0"/>')
+
+        self.expect("register read cpsr --format X", substrs=["cpsr = 0xEEEE7777"])
+        self.expect("register read cpsr --format X", substrs=["field_0"], matching=False)
+
     @skipIfXmlSupportMissing
     @skipIfRemote
     def test_xml_includes(self):