Work around spurious -Werror=array-bounds error in gcc-9.3.

Summary:
gcc-9.3 complains about folly::Format:

```
array subscript -1 is outside array bounds of ‘char [69]’
```

But the warning is incorrect; my guess is that it is confused because
it doesn't realize that higher-level logic guarantees that integer
formatting will run out of digits before it runs out of buffer.

Changing some code from `array + x` to `&array[x]` works around it.

Reviewed By: pixelb

Differential Revision: D20694399

fbshipit-source-id: 1176858054cdec9e415de4829371647958955103

folly/Format-inl.h

@@ -569,7 +569,7 @@ class FormatValue<
             presentation,
             "' specifier");
         valBufEnd = valBuf + valBufSize;
-        valBufBegin = valBuf + detail::uintToOctal(valBuf, valBufSize, uval);
+        valBufBegin = &valBuf[detail::uintToOctal(valBuf, valBufSize, uval)];
         if (arg.basePrefix) {
           *--valBufBegin = '0';
           prefixLen = 1;
@@ -582,7 +582,7 @@ class FormatValue<
             presentation,
             "' specifier");
         valBufEnd = valBuf + valBufSize;
-        valBufBegin = valBuf + detail::uintToHexLower(valBuf, valBufSize, uval);
+        valBufBegin = &valBuf[detail::uintToHexLower(valBuf, valBufSize, uval)];
         if (arg.basePrefix) {
           *--valBufBegin = 'x';
           *--valBufBegin = '0';
@@ -596,7 +596,7 @@ class FormatValue<
             presentation,
             "' specifier");
         valBufEnd = valBuf + valBufSize;
-        valBufBegin = valBuf + detail::uintToHexUpper(valBuf, valBufSize, uval);
+        valBufBegin = &valBuf[detail::uintToHexUpper(valBuf, valBufSize, uval)];
         if (arg.basePrefix) {
           *--valBufBegin = 'X';
           *--valBufBegin = '0';
@@ -611,7 +611,7 @@ class FormatValue<
             presentation,
             "' specifier");
         valBufEnd = valBuf + valBufSize;
-        valBufBegin = valBuf + detail::uintToBinary(valBuf, valBufSize, uval);
+        valBufBegin = &valBuf[detail::uintToBinary(valBuf, valBufSize, uval)];
         if (arg.basePrefix) {
           *--valBufBegin = presentation; // 0b or 0B
           *--valBufBegin = '0';