[NFC] [LLParser] Renaming LLParser routines to comply LLVM coding style

For any newly added parse function, clang-tidy complains. New parse
functions are implicitly defined by a macro "Parse##CLASS(N, IsDistinct)".
Now this macro and exising function definitions are corrected (lower case
first character). Some other variable/function names are also corrected
to comply LLVM coding style.

Reviewed By: djtodoro

Differential Revision: https://reviews.llvm.org/D90243