[Lex] Bring back the magic number 50 in updateConsecutiveMacroArgTokens.

This patch is a reland of 74e4f778 and
f83347b0 with the removed 50 trick back.

The magic number 50 was removed in D134942, as a behavior change for
performance reason.

While it reduces the number of SLocEntry, it increases the usage of
SourceLocation address space usage, which is critical for compiling
large TU.

This fixes a regression caused in D134942 -- clang failed to compile one of
our internal files, complaining the file is too large to process because clang
runs out of source location space (we spend 40% more address space!)

Differential Revision: https://reviews.llvm.org/D136539