Cache AllowAtInIdentifier as class variable in AsmLexer

This commit caches the value of the AllowAtInIdentifier variable as
a class variable in AsmLexer. We do this to avoid repeated MAI
queries and string comparisons each time we lex an identifier.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196622 91177308-0d34-0410-b5e6-96231b3b80d8