[lldb] Fix TestProcessIOHandlerInterrupt.py for macos

On darwin, we don't completely suppress the signal used to interrupt the
inferior. The underlying read syscall returns EINTR, which causes premature
termination of the input loop.

Work around that by hand-rolling an EINTR-resistant version of getline.

(cherry picked from commit a4d6de2)

Author: labath

lldb/test/API/iohandler/sigint/cat.cpp

@@ -1,9 +1,25 @@
-#include <iostream>
+#include <cstdio>
+#include <string>
+#include <unistd.h>
+
+std::string getline() {
+  std::string result;
+  while (true) {
+    int r;
+    char c;
+    do
+      r = read(fileno(stdin), &c, 1);
+    while (r == -1 && errno == EINTR);
+    if (r <= 0 || c == '\n')
+      return result;
+    result += c;
+  }
+}
 
 void input_copy_loop() {
   std::string str;
-  while (std::getline(std::cin, str))
-    std::cout << "read: " << str << std::endl;
+  while (str = getline(), !str.empty())
+    printf("read: %s\n", str.c_str());
 }
 
 int main() {