Misc fixes for LLVM.

Summary: - Update with LLVM build containing !locrec fixes.
- Do not assert when call instruction is not found for !locrec.
- Allow .rodata sections to be allocated at 16-byte boundary.

Reviewed By: @swtaarrs

Differential Revision: D1744740
Fix count_chars return value for empty strings

Summary: Closes #4395.
Closes https://github.com/facebook/hhvm/pull/4396

Reviewed By: @paulbiss

Differential Revision: D1721933

Signature: t1:1721933:1418866089:96ae64eefbcdd099751c2abaa9c35666c7066d40