LLVM currently emits dubious symbol sizes for aliases. E.g. assembling
the following with LLVM top-of-tree...

```
$ cat foo.s
        <snip>
        .data
        .globl  base
base:
        .dword  42
        .size   base, 8

.set alias, base+4
```

results in both base and alias having symbol size 8, even alias starts at
base + 4. This also means that alias extends past the end of the .data
section in this example.

We should probably teach LLVM not to do this in the future, but as a short-term
fix this patch teaches JITLink to simply truncate symbols that would extend
past the end of their containing block.

rdar://114207607
(cherry picked from commit 6267697)

Conflicts:
	llvm/lib/ExecutionEngine/JITLink/ELFLinkGraphBuilder.h

Swift is currently generating multiple .rodata sections with a combination of
SHF_ALLOC and (SHF_ALLOC | SHF_WRITABLE) flags and this was tripping an assert
in the ELFLinkGraphBuilder. As a temporary workaround this patch just uses the
union of the requested permissions.

rdar://114207428
(cherry picked from commit ead32e1)

Conflicts:
	llvm/lib/ExecutionEngine/JITLink/ELFLinkGraphBuilder.h