[PATCH] [IR/AsmWriter] Output escape sequences if the first character isdigit()

[Filipe Cabecinhas]

Hi dexonsmith, rafael,

If the first character in a metadata attachment's name is a digit, it has
to be output using an escape sequence, otherwise it's not valid text IR.

Removed an over-zealous assert from LLVMContext which didn't allow this.
The rule should only apply to text IR. Actual names can have any sequence
of non-NUL bytes.

Also added some documentation on accepted names.

Bug found with AFL fuzz.

http://reviews.llvm.org/D10079

Files:
  docs/LangRef.rst
  lib/IR/AsmWriter.cpp
  lib/IR/LLVMContext.cpp
  test/Feature/metadata.ll

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