[BOLT][llvm-lit] Enable internal shell for BOLT test suite

[connieyzhu]

Testing in lit is currently inconsistent across different platforms, as many RUN line commands are written in a way that only works on POSIX-compliant shells. Lit has an internal shell implementation to ensure uniform execution of these commands that can be used regardless of platform. We should aim to turn on the internal shell for the BOLT test suite to eliminate reliance on external shells.

The internal shell has missing capabilities that need to be addressed before doing so. Here is a list of the issues that block us from turning on the internal shell for BOLT.

• [llvm-lit] Unsupported |& redirection syntax in lit internal shell #102388
  [llvm-lit] lit internal shell fail to execute parentheses syntax #102401
  [llvm-lit] TypeError: string argument expected in BOLT when testing with lit internal shell #102693
  [llvm-lit] Attribute Error in lit internal shell #102399