Make sure IsATTY does not clobber errno.

Exposition of the problem:
> cat main.cpp

TEST(errnoTest, errnoTest)
{
    ASSERT_EQ(errno, 0);
}

int main(int argc, char** argv)
{
    ::testing::InitGoogleTest(&argc, argv);

    return RUN_ALL_TESTS();
}

Compiled with gcc 10 like this:
> g++ -pthread -o runtest main.cpp -Wl,-Bstatic -lgtest -Wl,-Bdynamic

Before patch:
>  ./runtest
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from errnoTest
[ RUN      ] errnoTest.errnoTest
[       OK ] errnoTest.errnoTest (0 ms)
[----------] 1 test from errnoTest (0 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test suite ran. (0 ms total)
[  PASSED  ] 1 test.

(output is colored, I run this inside an interactive terminal).

> ./runtest | cat
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from errnoTest
[ RUN      ] errnoTest.errnoTest
main.cpp:5: Failure
Expected equality of these values:
  (*__errno_location ())
    Which is: 25
  0
[  FAILED  ] errnoTest.errnoTest (0 ms)
[----------] 1 test from errnoTest (0 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test suite ran. (0 ms total)
[  PASSED  ] 0 tests.
[  FAILED  ] 1 test, listed below:
[  FAILED  ] errnoTest.errnoTest

 1 FAILED TEST

(output is not colored, since IsTTY return false, because of the pipe,
however it also clobbered errno for the tests).

After the patch, both cases are working fine:
> ./runtest
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from errnoTest
[ RUN      ] errnoTest.errnoTest
[       OK ] errnoTest.errnoTest (0 ms)
[----------] 1 test from errnoTest (0 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test suite ran. (0 ms total)
[  PASSED  ] 1 test.

> ./runtest | cat
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from errnoTest
[ RUN      ] errnoTest.errnoTest
[       OK ] errnoTest.errnoTest (0 ms)
[----------] 1 test from errnoTest (0 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test suite ran. (0 ms total)
[  PASSED  ] 1 test.

googletest/include/gtest/internal/gtest-port.h

@@ -252,6 +252,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <cerrno>
 #include <cstdint>
 #include <limits>
 #include <type_traits>
@@ -1960,16 +1961,16 @@ namespace posix {
 typedef struct _stat StatStruct;
 
 # ifdef __BORLANDC__
-inline int IsATTY(int fd) { return isatty(fd); }
+inline int DoIsATTY(int fd) { return isatty(fd); }
 inline int StrCaseCmp(const char* s1, const char* s2) {
   return stricmp(s1, s2);
 }
 inline char* StrDup(const char* src) { return strdup(src); }
 # else  // !__BORLANDC__
 #  if GTEST_OS_WINDOWS_MOBILE
-inline int IsATTY(int /* fd */) { return 0; }
+inline int DoIsATTY(int /* fd */) { return 0; }
 #  else
-inline int IsATTY(int fd) { return _isatty(fd); }
+inline int DoIsATTY(int fd) { return _isatty(fd); }
 #  endif  // GTEST_OS_WINDOWS_MOBILE
 inline int StrCaseCmp(const char* s1, const char* s2) {
   return _stricmp(s1, s2);
@@ -1994,7 +1995,7 @@ inline bool IsDir(const StatStruct& st) {
 typedef struct stat StatStruct;
 
 inline int FileNo(FILE* file) { return fileno(file); }
-inline int IsATTY(int fd) { return isatty(fd); }
+inline int DoIsATTY(int fd) { return isatty(fd); }
 inline int Stat(const char* path, StatStruct* buf) {
   // stat function not implemented on ESP8266
   return 0;
@@ -2011,7 +2012,7 @@ inline bool IsDir(const StatStruct& st) { return S_ISDIR(st.st_mode); }
 typedef struct stat StatStruct;
 
 inline int FileNo(FILE* file) { return fileno(file); }
-inline int IsATTY(int fd) { return isatty(fd); }
+inline int DoIsATTY(int fd) { return isatty(fd); }
 inline int Stat(const char* path, StatStruct* buf) { return stat(path, buf); }
 inline int StrCaseCmp(const char* s1, const char* s2) {
   return strcasecmp(s1, s2);
@@ -2022,6 +2023,18 @@ inline bool IsDir(const StatStruct& st) { return S_ISDIR(st.st_mode); }
 
 #endif  // GTEST_OS_WINDOWS
 
+inline int IsATTY(int fd)
+{
+    // DoIsATTY might change errno (for example ENOTTY in case you redirect stdout
+    // to a file on Linux), which is unexpected, so save the previous value, and
+    // restore it after the call.
+    int savedErrno = errno;
+    int isAttyValue = DoIsATTY(fd);
+    errno = savedErrno;
+
+    return isAttyValue;
+}
+
 // Functions deprecated by MSVC 8.0.
 
 GTEST_DISABLE_MSC_DEPRECATED_PUSH_()