[lldb] [test] Add a minimal test for x86 dbreg reading

Add a test verifying that after the 'watchpoint' command, new values
of x86 debug registers can be read back correctly.  The primary purpose
of this test is to catch broken DRn reading and help debugging it.

Differential Revision: https://reviews.llvm.org/D91264

lldb/test/Shell/Register/Inputs/x86-db-read.cpp

@@ -0,0 +1,12 @@
+#include <cstdint>
+#include <csignal>
+
+uint8_t g_8w;
+uint16_t g_16rw;
+uint32_t g_32w;
+uint32_t g_32rw;
+
+int main() {
+  ::raise(SIGSTOP);
+  return 0;
+}

lldb/test/Shell/Register/x86-db-read.test

@@ -0,0 +1,36 @@
+# Debug registers are currently printed only on FreeBSD.
+# REQUIRES: system-freebsd
+# REQUIRES: native && (target-x86 || target-x86_64) && dbregs-set
+# RUN: %clangxx_host -g %p/Inputs/x86-db-read.cpp -o %t
+# RUN: %lldb -b -s %s %t | FileCheck %s
+process launch
+# CHECK: Process {{[0-9]+}} stopped
+
+watchpoint set variable -w write g_8w
+# CHECK: Watchpoint created: Watchpoint 1: addr = 0x{{[0-9a-f]*}} size = 1 state = enabled type = w
+watchpoint set variable -w read_write g_16rw
+# CHECK: Watchpoint created: Watchpoint 2: addr = 0x{{[0-9a-f]*}} size = 2 state = enabled type = rw
+watchpoint set variable -w write g_32w
+# CHECK: Watchpoint created: Watchpoint 3: addr = 0x{{[0-9a-f]*}} size = 4 state = enabled type = w
+watchpoint set variable -w read_write g_32rw
+# CHECK: Watchpoint created: Watchpoint 4: addr = 0x{{[0-9a-f]*}} size = 4 state = enabled type = rw
+
+print &g_8w
+# CHECK: (uint8_t *) $0 = [[VAR8W:0x[0-9a-f]*]]
+print &g_16rw
+# CHECK: (uint16_t *) $1 = [[VAR16RW:0x[0-9a-f]*]]
+print &g_32w
+# CHECK: (uint32_t *) $2 = [[VAR32W:0x[0-9a-f]*]]
+print &g_32rw
+# CHECK: (uint32_t *) $3 = [[VAR64RW:0x[0-9a-f]*]]
+
+register read --all
+# CHECK-DAG: dr0 = [[VAR8W]]
+# CHECK-DAG: dr1 = [[VAR16RW]]
+# CHECK-DAG: dr2 = [[VAR32W]]
+# CHECK-DAG: dr3 = [[VAR64RW]]
+# CHECK-DAG: dr6 = 0x{{(00000000)?}}ffff0ff0
+# CHECK-DAG: dr7 = 0x{{(00000000)?}}fd7104aa
+
+process continue
+# CHECK: Process {{[0-9]+}} exited with status = 0