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

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.

llvm-svn: 238867