Eliminate more VLAs

Summary:
MSVC doesn't support them, so use dynamic allocation instead.
This also fixes a place where MSVC wasn't propogating the constexpr-ness of a local into the body of a lambda, resulting in an attempt to create a VLA.

Reviewed By: yfeldblum

Differential Revision: D4098932

fbshipit-source-id: 742b2fcd96f7f6eceb5043159403d557f7fc9673

folly/fibers/test/FibersTest.cpp

@@ -2057,7 +2057,7 @@ TEST(FiberManager, recordStack) {
     auto& loopController =
         dynamic_cast<SimpleLoopController&>(fm.loopController());
 
-    constexpr size_t n = 1000;
+    static constexpr size_t n = 1000;
     int s = 0;
     fm.addTask([&]() {
       int b[n] = {0};

folly/test/StringTest.cpp

@@ -1103,11 +1103,12 @@ char* copyWithSameAlignment(char* dst, const char* src, size_t length) {
 void testToLowerAscii(Range<const char*> src) {
   // Allocate extra space so we can make copies that start at the
   // same alignment (byte, word, quadword, etc) as the source buffer.
-  char controlBuf[src.size() + 7];
-  char* control = copyWithSameAlignment(controlBuf, src.begin(), src.size());
+  auto controlBuf = std::vector<char>(src.size() + 7);
+  char* control =
+      copyWithSameAlignment(controlBuf.data(), src.begin(), src.size());
 
-  char testBuf[src.size() + 7];
-  char* test = copyWithSameAlignment(testBuf, src.begin(), src.size());
+  auto testBuf = std::vector<char>(src.size() + 7);
+  char* test = copyWithSameAlignment(testBuf.data(), src.begin(), src.size());
 
   for (size_t i = 0; i < src.size(); i++) {
     control[i] = tolower(control[i]);