[libc] Add a functioning realloc for hermetic tests.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D150846

libc/test/UnitTest/HermeticTestUtils.cpp

@@ -74,9 +74,20 @@ void *malloc(size_t s) {
 
 void free(void *) {}
 
-void *realloc(void *ptr, size_t s) {
-  free(ptr);
-  return malloc(s);
+void *realloc(void *mem, size_t s) {
+  if (mem == nullptr)
+    return malloc(s);
+  uint8_t *newmem = reinterpret_cast<uint8_t *>(malloc(s));
+  if (newmem == nullptr)
+    return nullptr;
+  uint8_t *oldmem = reinterpret_cast<uint8_t *>(mem);
+  // We use a simple for loop to copy the data over.
+  // If |s| is less the previous alloc size, the copy works as expected.
+  // If |s| is greater than the previous alloc size, then garbage is copied
+  // over to the additional part in the new memory block.
+  for (size_t i = 0; i < s; ++i)
+    newmem[i] = oldmem[i];
+  return newmem;
 }
 
 // The unit test framework uses pure virtual functions. Since hermetic tests

libc/test/src/__support/CMakeLists.txt

@@ -95,8 +95,6 @@ add_libc_test(
 
 add_libc_test(
   char_vector_test
-  # This test relies on 'realloc' which is not implemented for hermetic tests.
-  UNIT_TEST_ONLY
   SUITE
     libc-support-tests
   SRCS

libc/test/src/__support/CPP/CMakeLists.txt

@@ -101,8 +101,6 @@ add_libc_test(
 
 add_libc_test(
   string_test
-  # This test relies on 'realloc' which is not implemented for hermetic tests.
-  UNIT_TEST_ONLY
   SUITE
     libc-cpp-utils-tests
   SRCS