[llvm][support] mustache library does not support escapes

[PeterChou1]

The mustache specification states that to specify an escape tag you can use a 'triple mustache', which stated here

https://mustache.github.io/mustache.5.html

the current implementation of the mustache spec in the support library does not support that and only supports escaping using the ampersand or &

this will cause certain compliance test in the spec to fail namely the following tests

Triple Mustache
Triple Mustache Integer Interpolation
Triple Mustache Decimal Interpolation
Triple Mustache Null Interpolation
Triple Mustache Context Miss Interpolation
Dotted Names - Triple Mustache Interpolation
Implicit Iterators - Triple Mustache
Triple Mustache - Surrounding Whitespace
Triple Mustache - Standalone
Triple Mustache With Padding
Standalone Indentation
Implicit Iterator - Triple mustache