These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

Static analysis identified the Len - 1 expression in
HeaderMapImpl::getString as problematic if Len is zero. After filing
this issue:

https://github.com/llvm/llvm-project/issues/130185

Indeed we should be checking MaxLen and verify it is not zero as well.

Fixes: https://github.com/llvm/llvm-project/issues/130185

Identified with misc-include-cleaner.

This patch adds an IsText parameter to the following getBufferForFile,
getBufferForFileImpl. We introduce a new virtual function
openFileForReadBinary which defaults to openFileForRead except in
RealFileSystem which uses the OF_None flag instead of OF_Text.

The default is set to OF_Text instead of OF_None, this change in value
does not affect any other platforms other than z/OS. Setting this
parameter correctly is required to open files on z/OS in the correct
encoding. The IsText parameter is based on the context of where we open
files, for example, in the ASTReader, HeaderMap requires that files
always be opened in binary even though they might be tagged as text.

This patch adds strnlen to the zOSSupport.h file to fix build failures in multiple files.