[analyzer] Retrieve a character from StringLiteral as an initializer for constant arrays.

Summary: Assuming that values of constant arrays never change, we can retrieve values for specific position(index) right from the initializer, if presented. Retrieve a character code by index from StringLiteral which is an initializer of constant arrays in global scope.

This patch has a known issue of getting access to characters past the end of the literal. The declaration, in which the literal is used, is an implicit cast of kind `array-to-pointer`. The offset should be in literal length's bounds. This should be distinguished from the states in the Standard C++20 [dcl.init.string] 9.4.2.3. Example:
  const char arr[42] = "123";
  char c = arr[41]; // OK
  const char * const str = "123";
  char c = str[41]; // NOK

Differential Revision: https://reviews.llvm.org/D107339