[clang-tidy] [modernize-use-auto] Add a threshold for minimal type name length...

[clang-tidy] [modernize-use-auto] Add a threshold for minimal type name length to be replaced with 'auto'

The threshold option is 'MinTypeNameLength' with default value '5'. 
With MinTypeNameLength == 5 'int'/'bool' and 'const int'/'const bool' 
will not be converted to 'auto', while 'unsigned' will be. 

Differential Revision: https://reviews.llvm.org/D45405

llvm-svn: 329730