Do not create LLVM IR `constant`s for objects with dynamic initialisation

When a const-qualified object has a section attribute, that
section is set to read-only and clang outputs a LLVM IR constant
for that object. This is incorrect for dynamically initialised
objects.

For example:

    int init() { return 15; }

    __attribute__((section("SA")))
    const int a = init();

a is allocated to a read-only section and is left
unintialised (zero-initialised).

This patch adds checks if an initialiser is a constant expression
and allocates objects to sections as follows:

* const-qualified objects
  - no initialiser or constant initialiser: .rodata
  - dynamic initializer: .bss
* non const-qualified objects
  - no initialiser or dynamic initialiser: .bss
  - constant initialiser: .data

(".rodata", ".data", and ".bss" names used just for explanatory
purpose)

Differential Revision: https://reviews.llvm.org/D102693