First step in supporting #line directives in assembler. This step parses the

First step in supporting #line directives in assembler.  This step parses the
#line directives with the needed support in the lexer.  Next will be to build
a simple file/line# table mapping source SMLoc's for later use by diagnostics.
And the last step will be to get the diagnostics to use the mapping for file
and line numbers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@139669 91177308-0d34-0410-b5e6-96231b3b80d8